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

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.

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}