Example to replicate the error message:
\documentclass{scrlttr2}
\usepackage{rotating}
\begin{document}
\begin{sidewaystable}
%\begin{tabular}{l}
%test \\
%\end{tabular}
\end{sidewaystable}
\end{document}\documentclass{scrlttr2}
\usepackage{rotating}
\begin{document}
\begin{sidewaystable}
%\begin{tabular}{l}
%test \\
%\end{tabular}
\end{sidewaystable}
\end{document}