Siunitx “duplicate-exponent-token” error in latex

Question

\begin{table}[ht]
\centering
\caption{Evaluation of the proposed }
\label{tab:Comput}
    \setlength{\tabcolsep}{0pt}
    \sisetup{round-mode=places,
             round-precision=2,
             table-format=1.2,
             }
\begin{tabular*}{\columnwidth}{@{\extracolsep{\fill}}  l  *{5}{S} }
    \toprule
 & \multicolumn{5}{c}{Proposed and benchmark forecasting frameworks} \\
    \cmidrule{2-6}
\thead{Months}
        & {\thead{FS-ANN\\ Computational \\efficiency}}
        & {\thead{Bi-level\\ Computational \\efficiency}}
            & {\thead{AFC-STLF\\ Computational \\efficiency}}
                & {\thead{MI-mEDE-ANN\\ Computational\\efficiency}}
                     & {\thead{FS-FCRBM-GWDO \\ Computational \\efficiency}} \\
\midrule
    Computational complexity (level) & Low & High & Moderate & High & Moderate \\

\midrule
%Avg.& 2.12 & 1.79 & 1.44 & 0.410\\
\bottomrule
\end{tabular*}
\end{table} 

Answer 1

As mentioned in comments: text in S columns should be inside curly braces. Since they are miising in texts in the last row, you got mentioned errors.

BTW, your table, as it is, is to wide that can be fit in text area (which is unknown, since you not provide MWE but just code fragment), so I suggest to make minor changes in table design:

\documentclass{article}
\usepackage{geometry}
\usepackage{booktabs, makecell}
\renewcommand\theadfont{\small\bfseries}
\renewcommand\theadgape{}
\newlength\colwidth
\usepackage[skip=1ex,labelfont=bf,font=small]{caption}
\usepackage{siunitx}

\begin{document}
\begin{table}[ht]
\centering
\caption{Proposed and benchmark forecasting frameworks}
\label{tab:Comput}
    \setlength{\tabcolsep}{0pt}
    \sisetup{round-mode=places,
             round-precision=2,
             table-format=1.2,
             }
    \settowidth\colwidth{complexity (level)}
\begin{tabular*}{\columnwidth}{ @{\extracolsep{\fill}}  
                                p{\colwidth}  *{5}{S} }
    \toprule
 & \multicolumn{5}{c}{\thead{Computational efficiency}} \\
    \cmidrule{2-6}
\thead{Months}
    & {\thead{FS-ANN}}
        & {\thead{Bi-level}}
            & {\thead{AFC-STLF}}
                & {\thead{MI-mEDE-ANN}}
                     & {\thead{FS-FCRBM-GWDO}} \\
\midrule
    Computational complexity (level) & {Low} & {High} & {Moderate} & {High} & {Moderate} \\

\midrule
Avg.& 2.12 & 1.79 & 1.44 & 0.410\\
\bottomrule
\end{tabular*}
\end{table} 
\end{document}

Answer 2

This should help

\begin{table}[ht]
    \centering
    \caption{Evaluation of the proposed }
    \label{tab:Comput}
    \setlength{\tabcolsep}{0pt}
    \sisetup{round-mode=places,
        round-precision=2,
        table-format=1.2,
    }
    \begin{tabular}{l*{5}{S}}                                                \toprule
                 & \multicolumn{5}{c}{Proposed and benchmark forecasting frameworks} \\                       \cmidrule{2-6}
        {Months} & {FS-ANN} & {Bi-level} & {AFC-STLF} & {MI-mEDE-ANN} & {FS-FCRBM-GWDO} \\         \cmidrule{1-6}
        {Computational complexity (level)} & {Low} & {High} & {Moderate} & {High} & {Moderate} \\   \cmidrule{1-6}
        Avg.& 2.12 & 1.79 & 1.44 & 0.410&\\                                                           \bottomrule
    \end{tabular}
\end{table}