It’s not possible to install an SSL Certificate on a subfolder. To enable SSL on a subfolder you can install an SSL Certificate on your root domain.
As soon as you have that installed, you also have SSL on your subfolder. The only thing left to do now is to activate SSL in Really Simple SSL.
If you want to enable SSL on https://root.com/subfolder/ you should install SSL on https://root.com/ and afterward activate SSL (with Really Simple SSL) on https://root.com/subfolder/
Concluding; installing SSL for your domain is based on below mapping: