Add new line to table cell in rmarkdown kable. Linebreak \n not working for latex output

I am creating a longtable in rmarkdown, which is text heavy. I'm setting the column width using column_spec to space text over multiple lines, but would like to insert manual line breaks to improve the readability. I have followed the advice here but still get an error saying the \n are not recognised:

! Undefined control sequence.
l.135 Aoike  2018 & H-Ex (12); \nCenter
                                       -Ex (13); \nCon (15) 

This is my code:

```{r setup, include=FALSE}
library(knitr)
knitr::opts_chunk$set(echo = FALSE)
library(readxl)
library(dplyr)
library(kableExtra)
book1 = read_excel("Table tests/Book1.xlsx", sheet = 1)
book1[is.na(book1)] <- " "
\begin{landscape}

```{r table,}
book1%>%
 mutate_all(linebreak)%>%
  kbl("latex", longtable = TRUE, escape = F, booktabs =  T, caption = '(ref:exstudies)')%>%
  column_spec(2, width= "5em")%>%
  kable_styling(latex_options= c("striped", "repeat_header"), font_size =10, repeat_header_method =  "replace")%>%
  footnote(general = "some text", general_title = "")
\end{landscape}

This is a mwe of my dataframe:

book1 = data.frame(Reference = c("Adams 2012", "Aoike 2018"),
                   `Groups (n)` = c("H-Ex (14); Con (15)", "H-Ex (12); \nCenter-Ex (13); \nCon (15)"))

Here is the contents of the .tex file:

% Options for packages loaded elsewhere
\PassOptionsToPackage{unicode}{hyperref}
\PassOptionsToPackage{hyphens}{url}
%
\documentclass[
  11pt,
]{article}
\usepackage{amsmath,amssymb}
\usepackage{lmodern}
\usepackage{ifxetex,ifluatex}
\ifnum 0\ifxetex 1\fi\ifluatex 1\fi=0 % if pdftex
  \usepackage[T1]{fontenc}
  \usepackage[utf8]{inputenc}
  \usepackage{textcomp} % provide euro and other symbols
\else % if luatex or xetex
  \usepackage{unicode-math}
  \defaultfontfeatures{Scale=MatchLowercase}
  \defaultfontfeatures[\rmfamily]{Ligatures=TeX,Scale=1}
  \setmainfont[]{Garamond}
\fi
% Use upquote if available, for straight quotes in verbatim environments
\IfFileExists{upquote.sty}{\usepackage{upquote}}{}
\IfFileExists{microtype.sty}{% use microtype if available
  \usepackage[]{microtype}
  \UseMicrotypeSet[protrusion]{basicmath} % disable protrusion for tt fonts
}{}
\makeatletter
\@ifundefined{KOMAClassName}{% if non-KOMA class
  \IfFileExists{parskip.sty}{%
    \usepackage{parskip}
  }{% else
    \setlength{\parindent}{0pt}
    \setlength{\parskip}{6pt plus 2pt minus 1pt}}
}{% if KOMA class
  \KOMAoptions{parskip=half}}
\makeatother
\usepackage{xcolor}
\IfFileExists{xurl.sty}{\usepackage{xurl}}{} % add URL line breaks if available
\IfFileExists{bookmark.sty}{\usepackage{bookmark}}{\usepackage{hyperref}}
\hypersetup{
  pdftitle={General Introduction},
  hidelinks,
  pdfcreator={LaTeX via pandoc}}
\urlstyle{same} % disable monospaced font for URLs
\usepackage[margin=1in]{geometry}
\usepackage{longtable,booktabs,array}
\usepackage{calc} % for calculating minipage widths
% Correct order of tables after \paragraph or \subparagraph
\usepackage{etoolbox}
\makeatletter
\patchcmd\longtable{\par}{\[email protected]\mbox{}\fi\par}{}{}
\makeatother
% Allow footnotes in longtable head/foot
\IfFileExists{footnotehyper.sty}{\usepackage{footnotehyper}}{\usepackage{footnote}}
\makesavenoteenv{longtable}
\usepackage{graphicx}
\makeatletter
\def\maxwidth{\ifdim\[email protected]@width>\linewidth\linewidth\else\[email protected]@width\fi}
\def\maxheight{\ifdim\[email protected]@height>\textheight\textheight\else\[email protected]@height\fi}
\makeatother
% Scale images if necessary, so that they will not overflow the page
% margins by default, and it is still possible to overwrite the defaults
% using explicit options in \includegraphics[width, height, ...]{}
\setkeys{Gin}{width=\maxwidth,height=\maxheight,keepaspectratio}
% Set default figure placement to htbp
\makeatletter
\def\[email protected]{htbp}
\makeatother
\setlength{\emergencystretch}{3em} % prevent overfull lines
\providecommand{\tightlist}{%
  \setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}
\setcounter{secnumdepth}{5}
\usepackage{pdflscape}
\usepackage{longtable}
\usepackage{lmodern}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{booktabs}
\usepackage{longtable}
\usepackage{array}
\usepackage{multirow}
\usepackage{wrapfig}
\usepackage{float}
\usepackage{colortbl}
\usepackage{pdflscape}
\usepackage{tabu}
\usepackage{threeparttable}
\usepackage{threeparttablex}
\usepackage[normalem]{ulem}
\usepackage{makecell}
\usepackage{xcolor}
\ifluatex
  \usepackage{selnolig}  % disable illegal ligatures
\fi

\title{General Introduction}
\author{}
\date{\vspace{-2.5em}}

\begin{document}
\maketitle

{
\setcounter{tocdepth}{2}
\tableofcontents
}
CKD exercise studies. See Table \ref{tab:exstudies}.



\begin{landscape}

\begingroup\fontsize{10}{12}\selectfont

\begin{longtable}[t]{>{\raggedright\arraybackslash}p{5em }>{\raggedright\arraybackslash}p{5em}>{\raggedright\arraybackslash}p{3em}>{\raggedright\arraybackslash}p{5em}>{\raggedright\arraybackslash}p{5em }>{\raggedright\arraybackslash}p{3em }>{\raggedright\arraybackslash}p{8em }>{\raggedright\arraybackslash}p{8em }>{\raggedright\arraybackslash}p{5em }>{\raggedright\arraybackslash}p{5em }}
\caption{\label{tab:exstudies}Exercise studies in patients with CKD. All studies in the table were randomised-controlled trials except Pechter\ldots{}}\\
\toprule
Study  Reference & Groups (n) & Study duration (weeks) & CKD Stage & Other population characteristics & Age (years) & Frequency/ duration & Aerobic & Resitance & Other elements of intervention\\
\midrule
\endfirsthead
\caption[]{\textit{(continued)}}\\
\toprule
Study  Reference & Groups (n) & Study duration (weeks) & CKD Stage & Other population characteristics & Age (years) & Frequency/ duration & Aerobic & Resitance & Other elements of intervention\\
\midrule
\endhead

\endfoot
\bottomrule
\multicolumn{10}{l}{\rule{0pt}{1em}All studies in the table were randomised-controlled trials except Pechter... }\\
\endlastfoot
\cellcolor{gray!6}{Aoike 2015} & \cellcolor{gray!6}{H-Ex (14); Con (15)} & \cellcolor{gray!6}{12} & \cellcolor{gray!6}{3-4} & \cellcolor{gray!6}{BMI>25} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{30-50 min., 3 x per wk} & \cellcolor{gray!6}{Walking; aiming for VT} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{Dietary instruction for all patients}\\
Aoike  2018 & H-Ex (12); \n Center Ex (13); \nCon (15) & 24 & 3-4 & Overweight & 56 & 40-60 min., 3 x per wk & Moderate (40-60\% VO2max): home-based = walking; centre-based = treadmill & Nil & \\
\cellcolor{gray!6}{Barcellos  2018} & \cellcolor{gray!6}{Ex (76); Con (74)} & \cellcolor{gray!6}{16} & \cellcolor{gray!6}{2-4} & \cellcolor{gray!6}{Hypertensive} & \cellcolor{gray!6}{65} & \cellcolor{gray!6}{60 min., 3 x per wk, supervised} & \cellcolor{gray!6}{Yes, no details} & \cellcolor{gray!6}{Yes, no details} & \cellcolor{gray!6}{}\\
Baria  2014 & Centre-Ex (10); H-Ex (8); Con (9) & 12 & 3-4 & BMI > 25; male & 52 & 30-50 min., 3 x per wk & Centre= treadmill; Home = walking\slash running; HR at VT & Nil & \\
\cellcolor{gray!6}{Beetham 2019} & \cellcolor{gray!6}{HIIT (9); Mod Ex (5)} & \cellcolor{gray!6}{12} & \cellcolor{gray!6}{3-4} & \cellcolor{gray!6}{1+  uncontrolled CVD risk factors (BP, HbA1C, lipids)} & \cellcolor{gray!6}{61} & \cellcolor{gray!6}{HIIT: 4x4 min.; Mod Ex: 40 min. 3 days per wk} & \cellcolor{gray!6}{HIIT: 80-95\% PeakHR, Mod Ex: 65\% peakHR; treadmill} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{}\\
\addlinespace
Castaneda  2001/2004 & Ex + low-protein diet (14); low-protein diet only (12) & 12 & SCr 133-442 $\mu$mol/L & Age >50 & 64 & 45 min., 3 x per wk & Nil & 8 reps x 3 sets of chest and leg press, latissimus pull-down, knee extension, and knee flexion & \\
\cellcolor{gray!6}{Eidemak  1997} & \cellcolor{gray!6}{Ex (15); Con (15)} & \cellcolor{gray!6}{Median 78} & \cellcolor{gray!6}{GFR 10-43} & \cellcolor{gray!6}{Non-diabetic} & \cellcolor{gray!6}{44} & \cellcolor{gray!6}{30 min., daily; unsupervised} & \cellcolor{gray!6}{60-75\% maximal exercise capacity, static bike, running, swimming or walking} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{}\\
Greenwood  2015 & Ex (8); Con (10) & 52 & 3-4 &  & 53 & 40 min., 3 x per wk (2 x supervised, 1 x home) & stationary exercise cycle; 80\% HRR & Free weights or therabands 80\% of 1RM & \\
\cellcolor{gray!6}{Headley  2012} & \cellcolor{gray!6}{Ex (10); Con (11)} & \cellcolor{gray!6}{48} & \cellcolor{gray!6}{2-4} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{55} & \cellcolor{gray!6}{45 min., 3 x per wk, supervised} & \cellcolor{gray!6}{50-60\% VO2 peak} & \cellcolor{gray!6}{Weeks 24-48; optional; weight machines} & \cellcolor{gray!6}{Dietary instruction for exercise group}\\
Headley  2014 & Ex (25); Con (21) & 16 & 3a-3b & DM or HTN as primary cause of CKD & 58 & 30-45 min., 3 x per wk, supervised & 50-60\% VO2 peak, mixed apparatus & Nil & \\
\addlinespace
\cellcolor{gray!6}{Hellberg 2019} & \cellcolor{gray!6}{Endurance \& strength (73); Endurance \& balance (75)} & \cellcolor{gray!6}{52} & \cellcolor{gray!6}{3-5} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{66} & \cellcolor{gray!6}{150 min./ wk} & \cellcolor{gray!6}{2 x 30 minutes; RPE = 13-15} & \cellcolor{gray!6}{3 x 30 minutes; RPE =13-17; 2-3 sets of 10 reps of wither free weight/body weight (resistance) or balance exercises} & \cellcolor{gray!6}{}\\
Hiraki  2017 & H-Ex (14); Con (14) & 52 & 3-4 & Male & 68.5 & 30 min. /day or 8000-10000 steps & Walking & Hand grip, squats and calf raises; 20-30 reps 3x per week & \\
\cellcolor{gray!6}{Howden  2013/2015} & \cellcolor{gray!6}{Ex \& lifestyle intervention (36); Con (36)} & \cellcolor{gray!6}{52} & \cellcolor{gray!6}{eGFR 25-60} & \cellcolor{gray!6}{1 or more uncontrolled CV risk factors (BP, HbA1C, lipids)} & \cellcolor{gray!6}{61} & \cellcolor{gray!6}{150 min./wk, beginning with 8 wks 2-3 x per wk supervised in gym; subsequently at home} & \cellcolor{gray!6}{Moderate, RPE 11-13} & \cellcolor{gray!6}{Whole-body with therabands and Swiss ball} & \cellcolor{gray!6}{Lifestyle intervention with dietician and psychologist (sustainable diet and behaviour change for weight loss)}\\
Ikizler  2018 & Ex vs usual activity; Calory restriction vs usual diet (104 total) & 16 & 3-4 & BMI> 25 & 53-58 & 30-45 min., 3 x per wk, supervised &  &  & 10-15\% calory reduciton\\
\cellcolor{gray!6}{Kirkman  2019} & \cellcolor{gray!6}{Ex (15); Con (16)} & \cellcolor{gray!6}{12} & \cellcolor{gray!6}{3-5} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{58} & \cellcolor{gray!6}{45 min., 3 x per wk; supervised} & \cellcolor{gray!6}{60-85\% heart rate reserve; cycling, treadmill or epliptical} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{}\\
\addlinespace
Kiuchi  2017 & HIIT (25); Mod Ex (25) & 156 & 3-4 &  & 58 & HIIT: 4-30 min.; Mod Ex: 30-60 min. 5 days per wk & HIIT: up to maximal intensity; Mod Ex: 55-85\% max HR & Nil & \\
\cellcolor{gray!6}{Leehey 2016} & \cellcolor{gray!6}{Ex + diet (14); Diet alone (18)} & \cellcolor{gray!6}{52} & \cellcolor{gray!6}{2-4} & \cellcolor{gray!6}{T2DM, obese, male} & \cellcolor{gray!6}{66} & \cellcolor{gray!6}{60 min., 3 x per wk; superveised for first 12 wks} & \cellcolor{gray!6}{45-85\% VO2 peak; Treadmill,elliptical or cycle; intervals} & \cellcolor{gray!6}{Additional 20-30 minutes/ session} & \cellcolor{gray!6}{Nutritional counselling session}\\
Leehey  2009 & Ex (7); Con (4) & 24 & 2-4 & Diabetic, BMI > 30, proteinuria & 66 & 30-40 min., 3 x per wk, supervised for first 6 wks & Walking; supervised sessions were up to 60-84\% VO2peak & Nil & \\
\cellcolor{gray!6}{Miele  2017} & \cellcolor{gray!6}{Ex (25); Con (21)} & \cellcolor{gray!6}{16} & \cellcolor{gray!6}{3} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{15-55 min., 3 x per wk, supervised} & \cellcolor{gray!6}{Moderate, 50-60\% VO2peak} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{}\\
Mustata  2011 & Ex (10); Con (10) & 52 & 3-4 &  & 68 & 20-60 min., 5 x per wk (3 unsupervised, 2 supervised) & 40-60\% VO2peak, treadmill, cycle, ellipitcal trainer; unsupervised = walking &  & \\
\addlinespace
\cellcolor{gray!6}{Pechter  2003} & \cellcolor{gray!6}{Ex (11); Con (9)} & \cellcolor{gray!6}{12} & \cellcolor{gray!6}{NA} & \cellcolor{gray!6}{Proteinuria; 2+ CV risk factors} & \cellcolor{gray!6}{49} & \cellcolor{gray!6}{30 min., 2x per wk} & \cellcolor{gray!6}{Aquatic exercise} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{}\\
Rossi  2014 & Ex (59); Con (48) & 12 & 3-4 &  & 68 & 60 min., 2 x per wk, supervised & Treadmill or static cycle; PLE<11(60-65\% HRM) & Free weights3 sets x 15 reps & Provided with pedometer\\
\cellcolor{gray!6}{Small 2017} & \cellcolor{gray!6}{Ex + lifestyle} & \cellcolor{gray!6}{52} & \cellcolor{gray!6}{eGFR 25-60} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{62} & \cellcolor{gray!6}{150 min./wk, beginning with 8 wks 2-3 x per wk supervised in gym; subsequently at home} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{4 weeks of group behavior and lifestyle modification and dietary advice}\\
Shi  2014 & Ex (11); Con (10) & 12 & NA & Evidence of CVD & 69 & 30 min., 3-5 x per wk, supervised for first 4 wks & Tai Chi & Nil & \\
\cellcolor{gray!6}{Tang  2017} & \cellcolor{gray!6}{H-Ex (45); Con (45)} & \cellcolor{gray!6}{12} & \cellcolor{gray!6}{1-3} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{45} & \cellcolor{gray!6}{30 min., 3 x per wk, at home} & \cellcolor{gray!6}{Walking, cycling jogging; 12-15 RPE} & \cellcolor{gray!6}{Nil} & \cellcolor{gray!6}{Exercise education (3 sessions)}\\
\addlinespace
Van Craenenbroeck  2015 & Ex (19); Con (21) & 12 & 3-4 &  & 53 & 10 min., 4x per day, at home & Static cycling, 90\% HR of anaerobic threshold & Nil & \\
\cellcolor{gray!6}{Huppertz 2020} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{} & \cellcolor{gray!6}{}\\
Zhou 2020 &  &  &  &  &  &  &  &  & \\*
\end{longtable}
\endgroup{}

\end{landscape}

\end{document}