I am using Willey Latex template. Please help me! multispan ->omit @multispan l.253 ...lticolumn{3}{c}{thead{24 h rehydration}}

If you are using the wiley-article replace thead by textbf to get

% !TeX TS-program = pdflatex

documentclass[12pt,a4paper]{wiley-article}

usepackage{threeparttable} % added <<<<<<<
usepackage{caption} % added <<<<<<<<<
begin{document}
    
begin{table}[!htp]
    caption{Two-way analysis of variance describing the influence of nitrogen (N) and water (W) 
        supply on several characteristics of amaranth plants. Significance levels are as in Table 1.}
    label{tab:tab2}
    centering
    begin{threeparttable}
    begin{tabular}{lcc ccc ccc}
        headrow
        multicolumn{1}{c}{textbf{Variables}} &  & multicolumn{3}{c}{ textbf{End of stress}} &  & multicolumn{3}{c}{ textbf{24 h rehydration}} 
                                                     &  &  N  & Water &      Ntextit{x}W      &  & N   & Water &       Ntextit{x}W        
        textit{g$_s$}                               &  & **  & ***   &          ***           &  & *** &  NS   &            NS             
        textit{E}                                   &  & NS  & ***   &          ***           &  & *** &  **   &             *             
        textit{A}                                   &  &  *  & ***   &          ***           &  & *** &   *   &            NS             
        textit{C$_i$}                               &  & *** & *     &           *            &  & NS  &  **   &            NS             
        WUE                                          &  & NS  & ***   &           **           &  & *** &  NS   &             *             
        Proline                                      &  & *** & *     &           *            &  & *** &  NS   &            NS             
        Leaf N                                       &  & *** & NS    &           NS           &  & *** &  NS   &            NS             
        Ammonium                                     &  & *** & NS    &           NS           &  & *** &  NS   &            NS             
        Nitrate                                      &  & *** & **    &           **           &  & *** &  NS   &            NS              hline
    end{tabular}
    end{threeparttable}
    end{table}
end{document}