Illegal parameter number in definition of \test. <to be read again> = l.343 \end{tabular} You meant to type ## instead of #, right? Or maybe a } was forgotten somewhere earlier, and things are all screwed up? I'm going to assume that you meant ##.
Solution:
Use #### instead of #.
Problem:
Overleaf gives this error when creating a table with double vertical lines.
% THIS DID NOT WORK \begin{table}[Summary Statistics of the analysis] \centering \begin{tabular}{|c||c|c|} \hline 1 & 2 & 3 \\ \hhline{|=#=|=|} 4 & 5 & 6 \\ \hline 7 & 8 & 9 \\ \hline \end{tabular} \end{table}
This part is the cause of the error: 1 & 2 & 3 \\ \hhline{|=#=|=|}
Instead, we should do: 1 & 2 & 3 \\ \hhline{|=####=|=|}
% THIS WORKED! \begin{table}[Summary Statistics of the analysis] \centering \begin{tabular}{|c||c|c|} \hline 1 & 2 & 3 \\ \hhline{|=####=|=|} 4 & 5 & 6 \\ \hline 7 & 8 & 9 \\ \hline \end{tabular} \end{table}