NB! I Corrected the \label[tab]{results}, which is wrong syntax unless your thesis-package redefines the label-command. However, I really recommend you to have a look at cleveref.
Use tabular X-column since you are using tabularx. In addition, I used a fixed width column as first column using array’s w-column (require an updated version of array). Also, use booktabs rules instead of hline and cline to have better spacing between the rows. If you prefer more space between row 2 and row 3, use \addlinespace from booktabs:
Example 1

\documentclass[12pt]{article}
\usepackage{tabularx, booktabs, array, caption}
\usepackage{multirow}
\begin{document}
\begin{table*}
\centering
\footnotesize
\caption{Parameter settings and results for different scenarios.
\label{tab:results}} %%%% <------- Corrected
\begin{tabularx}{\textwidth}{@{}>{\raggedright}wl{1.5cm}*{12}{>{\centering\arraybackslash}X}@{}}
\toprule
\multirow{5}{*}{\begin{tabular}[c]{@{}l@{}}Part\\ Type \#\end{tabular}} & \multicolumn{12}{c}{$\mu_j=U(91,876)$}\\ \cmidrule{2-13}
& \multicolumn{3}{c}{$\lambda_j=U(0.01,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.09,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.1,0.5)$} & \multicolumn{3}{c}{$\lambda_j=U(0.5,0.9)$} \\ \cmidrule{2-13}
& \multicolumn{12}{c}{Planning Horizon (Years)}\\ \cmidrule{2-13}
& 10& 20& 30& 10& 20& 30& 10& 20& 30& 10& 20& 30\\ \midrule
1& I& I& A& I& A& I& A& I& A& A& I& A\\
2& I& I& A& I& I& A& I& I& A& A& A& A\\
3& I& I& I& I& A& A& A& A& I& I& I& I\\
4& I& I& I& I& A& A& I& A& I& I& A& I\\
5& I& P& P& I& I& P& A& I& P& P& P& A\\
6& I& A& A& I& I& A& A& I& A& I& A& I\\ \bottomrule
\end{tabularx}
\end{table*}
\end{document}
Example 2
This is with split cmidrule and !{\hspace*{0.25em}} to increase space between column 4 and 5, 7 and 8 and 10 and 11 to group columns. In addition, I used \addlinespace to increase space between row 2 and 3.
It is the (l) statement in \cmidrule(l){2-4} that split the rule. Just remove the parenthesis and their content to have a solid rule.

\documentclass[12pt]{article}
\usepackage{tabularx, booktabs, array, caption}
\usepackage{multirow}
\begin{document}
\begin{table*}
\centering
\footnotesize
\setlength{\tabcolsep}{0.4em}
\caption{Parameter settings and results for different scenarios.
\label{tab:results}} %%%% <------- Corrected
\begin{tabularx}{\textwidth}{@{}>{\raggedright}
wl{1.5cm}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
@{}
}
\toprule
\multirow{6}{*}{\begin{tabular}[c]{@{}l@{}}Part\\ Type \#\end{tabular}}
& \multicolumn{12}{c}{$\mu_j=U(91,876)$}\\
\cmidrule(l){2-13}
& \multicolumn{3}{c}{$\lambda_j=U(0.01,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.09,0.1)$}
& \multicolumn{3}{c}{$\lambda_j=U(0.1,0.5)$} & \multicolumn{3}{c}{$\lambda_j=U(0.5,0.9)$} \\
\cmidrule(lr{\tabcolsep}){2-4}\cmidrule(lr{\tabcolsep}){5-7}
\cmidrule(lr{\tabcolsep}){8-10}\cmidrule(l){11-13}
\addlinespace[3pt]
& \multicolumn{12}{c}{Planning Horizon (Years)}\\ \cmidrule(l){2-13}
& 10& 20& 30& 10& 20& 30& 10& 20& 30& 10& 20& 30\\ \midrule
1& I& I& A& I& A& I& A& I& A& A& I& A\\
2& I& I& A& I& I& A& I& I& A& A& A& A\\
3& I& I& I& I& A& A& A& A& I& I& I& I\\
4& I& I& I& I& A& A& I& A& I& I& A& I\\
5& I& P& P& I& I& P& A& I& P& P& P& A\\
6& I& A& A& I& I& A& A& I& A& I& A& I\\ \bottomrule
\end{tabularx}
\end{table*}
\end{document}
utthesis.clsfrom? – leandriis Oct 07 '19 at 21:23\label[tab]{results}is supposed to do. – leandriis Oct 07 '19 at 21:27tabularxbut you have noXcolumns in your table. What do you expect it to do? – Alan Munn Oct 07 '19 at 21:30tabular*rather than atabularxenvironment. – Mico Oct 07 '19 at 21:32\multicolumnentries are longer than the combined widths of the three corresponding columns. For more information see here: Table column widths disproportionate due to multicolumn cell being too long. Probably you could introduce linebreaks in the column headers to circumvent this problem. I'd also recommend that you take a look at the horizontal lines from thebooktabspackage. – leandriis Oct 07 '19 at 21:32\label[tab]{results}just defines label for cleveref. – tcokyasar Oct 07 '19 at 21:36@{\extracolsep{\fill}}doesn't belong in the picture at all. You should be using suitably modified versions of theXcolumn type instead. – Mico Oct 07 '19 at 21:40