"Something's wrong--perhaps a missing \item" when using longtable

Question

I have to make a table that is quite long so I use the longtable package. The following code gives me the Something's wrong--perhaps a missing \item error for the \begin{center} line.

\documentclass[a4paper, 12pt]{article}
\usepackage[frenchb]{babel}
\usepackage[T1]{fontenc}
\usepackage{float}
\usepackage{caption}
\usepackage{xltabular}
\usepackage{longtable}
\usepackage{hyperref}


        \begin{longtable}[H]{c|c|c|c|c|c}
            \begin{center}
                    \hline
                    \textbf{ID projet} & \textbf{Nom du projet} & \textbf{Complexité} & \textbf{Disponibilité des données} & \textbf{Coût} & \textbf{Impact}  \\
                    \hline
                    cell & cell  & cell & cell  & cell & cell  \\
                    \hline
                    cell  & cell  & cell & cell & cell &cell \\   
                    \hline
                    cell & cell & cell & cell & cell \\
                    \hline
                    cell & cell & cell &  cell & cell & cell \\
                    \hline
                    cell & cell & cell& cell & cell & cell \\
                    \hline
                    cell & cell & cell & cell & cell & cell \\
                    \hline
                    cell & cell & cell & cell & cell & cell \\
                    \hline
            \end{center}
        \end{longtable}

I don't know if it's related but the first \hline gives me the Misplaced \noalign error and the \textbf line gives me the Misplaced \omit error.

Answer 1

You must not hide the content of your longtable inside a center environment. If you want the table to be centred, you can use the optional [c] argument. However this won't make a visual difference because your table does not fit within the available text width.

\documentclass[a4paper, 12pt]{article}
\usepackage[french]{babel}
\usepackage[T1]{fontenc}
\usepackage{float}
\usepackage{caption}
\usepackage{xltabular}
\usepackage{longtable}
\usepackage{hyperref}

\begin{document}

        \begin{longtable}[c]{c|c|c|c|c|c}
                    \hline
                    \textbf{ID projet} & \textbf{Nom du projet} & \textbf{Complexité} & \textbf{Disponibilité des données} & \textbf{Coût} & \textbf{Impact}  \\
                    \hline
                    cell & cell  & cell & cell  & cell & cell  \\
                    \hline
                    cell  & cell  & cell & cell & cell &cell \\   
                    \hline
                    cell & cell & cell & cell & cell \\
                    \hline
                    cell & cell & cell &  cell & cell & cell \\
                    \hline
                    cell & cell & cell& cell & cell & cell \\
                    \hline
                    cell & cell & cell & cell & cell & cell \\
                    \hline
                    cell & cell & cell & cell & cell & cell \\
                    \hline
        \end{longtable}

        \end{document}