I tried to type % in my table, but its overlaps.

[![enter image description here](https://i.stack.imgur.com/AsdWW.png)](https://i.stack.imgur.com/AsdWW.png)

```
\documentclass[12pt,oneside]{book}

\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}

\usepackage{makecell,siunitx}
\usepackage{booktabs}

\begin{document} 

\begin{table}[h!]
\centering
\sisetup{table-format=3.0, table-number-alignment=left, table-column-width=2.0cm}
 \begin{tabular}{lSS}
    \toprule
    & \multicolumn{2}{c}{\thead{\makebox[0pt]{Testing 123 (\%)}}}\\ 
     \cmidrule{2-3}
    &{\small \textbf{Accuracy}}
     & {\small\textbf{Inaccuracy}} \\
     \midrule
    N       & 20    & 20 \\
    Mean    & 50.45\%  & 58.55\% \\
    Median  & 78.74\% & 665.26\% \\
        \bottomrule
    \end{tabular}
\end{table}

\end{document}

```