Longtabu is not supported

[IgniparousTempest]

\usepackage{longtable}
\usepackage{tabu}

\begin{longtabu} to \textwidth {
    X[1,c]
    X[1,c]
    X[1,c]
    X[10,l]}
    \hline \hline
    \textbf{Time End of Step} & 
    \textbf{Step} & 
    \textbf{Event} &
    \textbf{Description} 
    \\ \hline \hline \hline \hline
    12:18:52 & 1 & 1 & this is a description for the event \\
    12:18:52 & 1 & 1 & this is a description for the event \\
    12:18:52 & 1 & 1 & this is a description for the event \\
\end{longtabu}

The above is a valid longtabu table.

[JDMCreator]

Confirmed. It will be fixed in the next minor release, even though tabu is no longer recommanded by the LaTeX3 Project Team.