If you see this message when you access your site over https:
To change this page upload a new index.html to your private_html folder
This means there are different folders for http:// and https://, your site is in the http (public_html) and you’re looking at the private_html folder.
The site will load fine over http:// and Really Simple SSL will be able to detect SSL (it’s installed correctly after all), but activation will make your site unaccessible because it points to this wrong directory.
You can check if this is the case by visiting the https:// domain of your site. This is usually fixed by enabling the “Use a symbolic link from private_html to public_html” in DirectAdmin. Setting a symbolic link will make the content of the https:// folder the same as http://. See also this thread: https://forum.directadmin.com/showthread.php?t=38843