Where is the class file cjs-rcs-article.cls?

If you have an up-to-date TeX distribution, the class file cjs-rcs-article.cls will already be installed on your system through the package cjs-rcs-article and is ready for use. You can check this with the following command issued from a command line:

kpsewhich cjs-rcs-article.cls

The above also applies to online editors such as Overleaf, since their TeX distributions are usually up-to-date.

If a system installation of the class isn’t suitable, consider one of the alternative installation scenarios.