安装错误“paracol”包

Question

当我安装“paracol”包时，出现以下消息“未找到文件 paracol.sty”。也许我犯了一些错误，但我没有检测到它。

\documentclass{letter}
\usepackage{paracol}

\begin{document}

\columnratio{0.6}
\begin{paracol}{2}
    \begin{leftcolumn}
        The environment may also be restarted
        anywhere you like as shown here. ...
    \end{leftcolumn}
    \begin{rightcolumn}
        source
    \end{rightcolumn}
\end{paracol}
Now the aurthor will do ...

\end{document}