The line says: misplaced \omit. underfull \hbox badness(1000) in alignment at line 818--818. Also the error is mentioned below

Question

The Code I ran:

\begin{table*}[htbp]
\caption{alekga g. The number of stars represents the strength levels. (* = good, * = better, * = best)}
\centering
\begin{tabularx}{\textwidth}{|>{\centering\hsize=0.3\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.05\hsize\arraybackslash}X|>{\centering\hsize=0.35\hsize\arraybackslash}X|}
     
\hline
 &
\multicolumn{7}{c|}{\textbf{Strengths}} &
   \\ \cline{2-8}
\multirow{-2}{*}{\textbf{Techniques}} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{Complexity }} \end{turn} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{Accuracy}} \end{turn} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{Robustness }} \end{turn} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{VSTLF}} \end{turn} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{STLF}} \end{turn} & \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{MTLF}} \end{turn}& \begin{turn}{90}
  \multicolumn{1}{l}{\textbf{LTLF}} \end{turn} &
  \multirow{-2}{*}{\textbf{References}} \\ \hline
LSTM &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{X} &
  X &
  \cite{Rafi2021,Phyo2019221,dat_hybrid_2021,YANG2022108150,Han2019,Fang2022,JAVED2022117689,JHA2021107479,SHI1,GUO20201046,Xuan2021} \\ \hline
SVR &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{*} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{***} &
  *** &
  \cite{KAVOUSIFARD20146047,WANG201265,en5114430,en9020070,FattaheianDehkordi2014,Tran20223410,KONG2020114368,Niu_2021,ZHANG2012850,KAYTEZ2015431,Alire2019,Li2016,Zhang2020,Zeng2018,en9100827,Jawad2020} \\ \hline
ANN &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{***} &
  *** &
  \cite{Mohammed2022,article11,Hamzacebi2019,OFORINTOWJNR2021102679,su14010188,rizk2020,WU2013126,arssfdticle,Kavousi-Fard2014517,golilarz_short-term_2021,Farfar2019,Phyo2021,kuruge,Gafar2018,essallah_comparative_2019,YU2015176,ARDAKANI2014452,AZADEH2013605,GUO20181186,Cheng2020,salgado_hybrid_2013,Liu2021,Gao2020,MOAZZAMI2013489,Ma2020,articsdale,YU2014102,BENDAOUD2021111152,en12122445,sarvba,en14175510,Sheikhan2012,azadeh_optimization_2014,houimli_short-term_2020,hu020,Mir2021,liu_short-term_2021,ARVV3,electronics11101524,Ananthu2021114,Bedi2018} \\ \hline
Time series &
  \multicolumn{1}{l|}{*} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{*} &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{**} &
  *** &
  \cite{saleh_hybrid_2014,KAZEMZADEH2020117948,fard201223,VELASQUEZ2022123483,paperi021,Xie2017,LEE2012251,HAMZACEBI2014165,zeng2017,YUKSELTAN2020100524,en13205309,ZHAO2016272,paul2019,Han2021,Li2018,CHRENG202276,SERRANOGUERRERO2021117173,migon2013,9403196} \\ \hline
ANFIS &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{**} &
  \multicolumn{1}{l|}{*} &
  \multicolumn{1}{l|}{***} &
  \multicolumn{1}{l|}{X} &
  ** &
  \cite{SHAO2015876,Fallahpour2021142,mollaiy-berneti_optimal_2016,jain2019,PatiRaySingh+2022+59+72,LAOUAFI2016136,Cevik2015,artitiancle,rahtor20} \\ \hline
  \end{tabularx}
  \label{st4}
  \end{table*}

The error i got:

\multispan ->\omit 
                   \@multispan 
l.818   \end{tabularx}
                      
I expect to see \omit only after tab marks or the \cr of
an alignment. Proceed, and I'll ignore this case.

Answer 1

Your construction of rotated cell is erroneous. Instead of

\begin{turn}{90} \multicolumn{1}{l}{\textbf{<content>}} \end{turn} 

you should write simple

\begin{turn}{90} \textbf{<content>}\end{turn} 

With this corrections your table code can be compiled without errors. Result is:

Off-topic:

• Your table seems to be generator with some table generator ... consequently code contain a lot unnecessary \multicolumn{1}{l|}{...}. Remove all them.
• The width of the first column is too wide! Try to change its column specification to c or l.
• Your column specification is overwritten by commands \multicolumns in table code. Consequently cells contents are left aligned instead to be centered.
• To my opinion nicer table with far simpler code you will get by use of the `tabularray package:

\documentclass[twocolumn]{article}
\usepackage{rotating}
\usepackage{makecell}
\usepackage{tabularray}

\begin{document}
    \begin{table*}
\caption{alekga g. The number of stars represents the strength levels.
         (* = good, * = better, * = best)}
\label{st4}
\settowidth\rotheadsize{\normalsize\bfseries Complexity}
\renewcommand\theadfont{\normalsize}
\begin{tblr}{hlines, vlines,
             colspec = {l *{7}{X[c]} X[7,l]},
             row{1}  = {font=\bfseries},
             row{2}  = {font=\bfseries, cmd=\rothead}
             }
\SetCell[r=2]{c}  Techniques
    &    \SetCell[c=7]{c}  Strengths
        &   &   &   &   &   &   &  \SetCell[r=2]{c}  References \\
    & Complexity
        & Accuracy
            & Robustness
                & VSTLF
                    &  STLF
                        &  MTLF
                            &  LTLF
                                &       \\
LSTM    & *** & **  & *** & *** & *** & X   & X
    & \cite{Rafi2021, Phyo2019221, dat_hybrid_2021, YANG2022108150, Han2019, Fang2022,
            JAVED2022117689, JHA2021107479, SHI1, GUO20201046, Xuan2021} \\
SVR     & **  & *** & **  & *   & *** & *** & ***
    & \cite{KAVOUSIFARD20146047,WANG201265,en5114430,en9020070,FattaheianDehkordi2014,
            Tran20223410,KONG2020114368,Niu_2021,ZHANG2012850,KAYTEZ2015431,Alire2019,
            Li2016,Zhang2020,Zeng2018,en9100827,Jawad2020} \\
ANN     & **  & *** & *** & **  & ***  & *** & ***
    & \cite{Mohammed2022,article11,Hamzacebi2019,OFORINTOWJNR2021102679,su14010188,rizk2020,
            WU2013126,arssfdticle,Kavousi-Fard2014517,golilarz_short-term_2021,Farfar2019,
            Phyo2021,kuruge,Gafar2018,essallah_comparative_2019,YU2015176,ARDAKANI2014452,
            AZADEH2013605,GUO20181186,Cheng2020,salgado_hybrid_2013,Liu2021,Gao2020,MOAZZAMI2013489,
            Ma2020,articsdale,YU2014102,BENDAOUD2021111152,en12122445,sarvba,en14175510,Sheikhan2012,
            azadeh_optimization_2014,houimli_short-term_2020,hu020,Mir2021,liu_short-term_2021,
            ARVV3,electronics11101524,Ananthu2021114,Bedi2018} \\
Time series
        & *   & *** & *   & **  & *** & **  & ***
    & \cite{saleh_hybrid_2014,KAZEMZADEH2020117948,fard201223,VELASQUEZ2022123483,paperi021,
            Xie2017,LEE2012251,HAMZACEBI2014165,zeng2017,YUKSELTAN2020100524,en13205309,
            ZHAO2016272,paul2019,Han2021,Li2018,CHRENG202276,SERRANOGUERRERO2021117173,
            migon2013,9403196} \\
ANFIS   & **  & *** & **  & *   & *** & X   & **
    & \cite{SHAO2015876,Fallahpour2021142,mollaiy-berneti_optimal_2016,jain2019,
            PatiRaySingh+2022+59+72,
            LAOUAFI2016136,Cevik2015,artitiancle,rahtor20} \\
\end{tblr}
    \end{table*}
\end{document}