I have the following longtabu which does not compile and give an error. This example is shorter than my original, but my table just continues with more columns.
\documentclass{standalone}
\usepackage{amsthm,amsmath}
\RequirePackage{natbib}
\RequirePackage[authoryear]{natbib}% uncomment this for author-year bibliography
\RequirePackage{hyperref}
\usepackage[utf8]{inputenc} %unicode support
\urlstyle{same}
\usepackage[width=\textwidth]{caption}
\captionsetup{
labelsep = space,
justification = raggedright,
font = {footnotesize,singlespacing,sf},
labelfont = {footnotesize,bf,singlespacing,sf},
singlelinecheck=off,
skip=4pt,
position=top}
\usepackage{float}
\usepackage{pgfplots}
\usepackage{graphicx}
\usepackage{booktabs}
\usepackage{tabularx}
\usepackage{longtable,tabu}
\usepackage{array}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}}
\newcolumntype{R}[1]{>{\raggedleft\arraybackslash}p{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\usepackage{threeparttable}
\usepackage{pgfplotstable}
\usepackage[markup=nocolor,final]{changes} Annehmen der Änderungen
\usepackage[markup=nocolor]{changes}
\usepackage{longtable,tabu}
\begin{document}
\begin{longtabu} to \textwidth{XXXX}
\caption{Questions of online questionnaire}\\
\toprule
\multicolumn{1}{X}{Question Nr} & \multicolumn{1}{p{5cm}}{Questions} & \multicolumn{1}{X}{Answer choices (simplified)} & \multicolumn{1}{X}{Conditions}\\
\midrule
1 & \multicolumn{1}{p{5cm}}{Is your practice part of practice network consisting of practices at different places?} & Yes/No (Y/N) & Madatory for all \\
2 & \multicolumn{1}{p{5cm}}{How many doctors do work in your practice?} & Number & Madatory for all \\
\end{longtabu}
\end{document}
The error is the following:
Argument of \tabu@ has an extra}
Any help is appreciated! Thanks :)

\end longtabushould be replaced by\end{longtabu}. – leandriis Jun 15 '18 at 15:56\multicolumn{1}commands? They seem unnecessary to me. Also using theXcolumn type for all 4 columns is not needed. You could for example use{p{1.2cm}Xp{2.5cm}l}instead. – leandriis Jun 15 '18 at 16:00