I read many threads on this forum and spent a lot of time to solve this problem, unfortunately, no matter how much I try, I haven’t fixed it yet. I could draw the table below before, with my latex codes. My aim is to draw the same table again.
But now, by using the same codes shown below, I have received this error:
“! Misplaced \omit. \multispan ->\omit “
\begin{table}\centering \begin{adjustbox}{width=0.45\textwidth} \begin{tabular}{@{}rrrc|rrr|cr|@{}} \hline \\ [-4ex] & \multicolumn{3}{c}{\textbf{Mix Rate}} & \multicolumn{3}{c}{\textbf{Mutation factor}} & \multicolumn{2}{c}{\textbf{Crossover probability}} \\ \textbf{Dimensions} & $Mean$ & $Std$ && $Mean$ & $Std$ && $Mean$ & $Std$ \\ \midrule[0.5pt] \multicolumn{1}{l}{\textbf{30}} & 0.550 & 0.091 && 0.519 & 0.089 && 0.599 &0.133 \\ \multicolumn{1}{l}{\textbf{50}} & 0.549 & 0.076 && 0.521 & 0.094 && 0.565 &0.125 \\ \multicolumn{1}{l} {\textbf{100}} & 0.609 & 0.085 && 0.529 & 0.093 && 0.504 &0.135 \\ \bottomrule \end{tabular} \end{adjustbox} \end{table} I would really appreciate it if anyone could help me to fix this problem. Many thanks in advance.

\\\in your code, while\\is the correct one.\\\is parsed as two commands\\and\<space>, where the latter expands to a fixed size space character. This breaks the following\multicolumncommand, as this has to be (if used at all) the very first thing in a tabular cell.adjustboxandbooktabs. Could you please extend your code to include your preamble as well, i.e. make it a full MWE?