Loading the datetime2 package with the showdow option creates a clash with the ctable package, at least for me. Without the showdow option there are no problems. The following MWE shows the error message and the output as comments in lines 12 and 13.
% !TeX program = lualatex
% !TEX encoding = UTF-8 Unicode
% !TeX spellcheck = en_US
\documentclass{article}
\usepackage{ctable}
\usepackage[showdow,en-US]{datetime2}
%\usepackage[en-US]{datetime2} causes no problem
\begin{document}
Month: \DTMmonthname{2}.
%Error line 11: Undefined control sequence. \begin{document}
%Output: /TRP1«/ca1/CA 1» Month: February.
\end{document}