\documentclass{report}
\usepackage{multirow}
\begin{document}
\begin{table}
{\begin{tabular}[l]{@{}lcccccccccc}\hline
n=60,p&Method &$F$&$L$
& $Elastic-Net$ & $Ridge$ & $SCAD$&$OSC$& $\widehat{\Re}_{diagonal}$ & $\widehat{\Re}_{\widehat{%
         \Del}}$ & $\widehat{\Re}_{spider}$ \\
         \hline
\multirow{4}{*}{p=400} &RMS &-&- & - & - & -&- &- &  - &-\\
&Number of effective variables &10 &40 & 47 & 400 & 400 &-& 400 & 400 & 400\\ 
&Dimension of the central subspace & -&- & - &- & - & -& 1& 1& 1 \\
&Computation time (second) &19.126&2.590& 1.903 & 7.816 & 2950.382 &-&  0.702 & 471.448 & 4922.220\\
\hline
\multirow{4}{*}{p=56} &RMS&- &-&- & - & -&-&-& -&-\\
&Number of effective variables &-&17 & 19 & 56 & 56 & 56& 56 & 56 & 56 \\ 
&Dimension of the central subspace &-& - & - &- & - &-&  2& 2& 2 \\
&Computation time (second) & -& 0.158 & 0.219 &0.297 & 6.562 &5825.186&  0.141 & 14.484 & 16.922\\
\end{tabular}}\label{sim}
\end{table}
\end{document}