Converting Table to Longtable

Question

I tried to convert a table to longtable but did not succeed.

\documentclass[a4paper,12pt]{report}

\usepackage[usenames,dvipsnames,svgnames,table]{xcolor}
\usepackage[T1]{fontenc}
\usepackage{natbib}
\usepackage[francais]{babel}
\usepackage{url} % Pour écrire des adresses cliquables.
\usepackage{lmodern} % Pour changer le pack de police.
\usepackage[top=2.5cm, bottom=2.5cm, left=2.5cm, right=2.5cm]{geometry}
\usepackage{fancyhdr}
\usepackage{supertabular} % tableaux qui tiennent sur plusieurs pages

\usepackage{array}
\usepackage{pdflscape}
\usepackage[utf8x,utf8]{inputenc}

%\usepackage[numbers]{natbib}
%\usepackage[square]{natbib}
\usepackage{soul} %alignier


\usepackage{epigraph}
\usepackage{setspace}
\usepackage{float}
%\linespread{1.15}
\onehalfspacing
\usepackage{makecell}
\usepackage{tabularx}
\usepackage{amsmath}
\usepackage{algorithm}
\usepackage{algpseudocode}
\usepackage{blindtext}
\usepackage{enumitem}
\usepackage{verbatim}
\usepackage{tikz}
\bibliographystyle{abbrvnat1}
%\bibliographystyle{apalike}
\setcitestyle{authordate,open={(},close={)}}


\begin{document}

\begin{table}[H]
    \centering
    \label{CU1-label}
\begin{tabularx}{\textwidth}{|X|}%{|X|}
\hline          
    \textbf{Cas d'utilisation: XXX} \\
    \hline          
    \textbf{Description:}    
        \\ \textbf{Titre}: 
      \\ \textbf{But}: XXX
      \\ \textbf{Acteur}: XXX.
      \\\hline

      \textbf{Pré-conditions}:     \\\hline
      \textbf{Enchaînements nominaux}:
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ \hline \textbf{Exceptions}:
      \\ XXX
      \\ \hline \textbf{Post-conditions}:
      \\ XXX
      \\
    \hline      
\end{tabularx}
    \caption{Cas d'utilisation: XXX}

\end{table}

\end{document}

I tried many solution online but they don't seem to work, what I am getting is:

LaTeX Error: Can be used only in preamble. You have used a command in the main body of your document which should be used in the preamble. Make sure that \documentclass[…]{…} and all \usepackage{…} commands are written before \begin{document}.

How can I fix this?

Answer 1

The conversion of this table into a breakable table is very easy using package ltablex:

\documentclass[a4paper,12pt]{report}

\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage{lmodern} % Pour changer le pack de police.
\usepackage[utf8]{inputenc}% combination of utf8x and utf8 does not make sense

\usepackage{ltablex}
\keepXColumns % see ltablex manual

\begin{document}

\begin{tabularx}{\textwidth}{|X|}%{|X|}
    \caption{Cas d'utilisation: XXX}
    \label{CU1-label}
\endfirsthead
\hline          
    \textbf{Cas d'utilisation: XXX} \\
    \hline          
    \textbf{Description:}    
        \\ \textbf{Titre}: 
      \\ \textbf{But}: XXX
      \\ \textbf{Acteur}: XXX.
      \\\hline

      \textbf{Pré-conditions}:     \\\hline
      \textbf{Enchaînements nominaux}:
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ \hline \textbf{Exceptions}:
      \\ XXX
      \\ \hline \textbf{Post-conditions}:
      \\ XXX
      \\
    \hline      
\end{tabularx}

\end{document}

You could also use ltxtable. The solution would be almost equal:

\documentclass[a4paper,12pt]{report}

\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage{lmodern} % Pour changer le pack de police.
\usepackage[utf8]{inputenc}% combination of utf8x and utf8 does not make sense

\usepackage{ltxtable}
\usepackage{filecontents}

\begin{document}

\begin{filecontents}{\jobname-table.tmp}
\begin{longtable}{|X|}
    \caption{Cas d'utilisation: XXX}
    \label{CU1-label}
\endfirsthead
\hline          
    \textbf{Cas d'utilisation: XXX} \\
    \hline          
    \textbf{Description:}    
        \\ \textbf{Titre}: 
      \\ \textbf{But}: XXX
      \\ \textbf{Acteur}: XXX.
      \\\hline

      \textbf{Pré-conditions}:     \\\hline
      \textbf{Enchaînements nominaux}:
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ - XXX
      \\ \hline \textbf{Exceptions}:
      \\ XXX
      \\ \hline \textbf{Post-conditions}:
      \\ XXX
      \\
    \hline      
\end{longtable}
\end{filecontents}
\LTXtable{\textwidth}{\jobname-table.tmp}

\end{document}

Note, I've removed all packages that are not needed to make your table to make the example more minimal.

BTW: This does not really look like a table. Maybe just using some boxes would be a better solution. Perhaps package tcolorbox could be useful too.