When i try to enable autoSSL for a particular domain, it does not install the certificate. It seems that some file inside the folder ".well-known" does not exist or is this same. please help.
Your .htaccess rules seem to be preventing the certificate from installation. Try adding the following code to the top of the .htaccess file in "public_html" folder:
Check your .htaccess file for redirects. If there are complete matches for the entire domain, this won't be compatible with the Let's Encrypt challenge.
If it's not within your .htaccess file, within Plesk go to the domain and then Apache and nginx Settings.
If you have a blanket redirect, remove it and add it using the following guide: Plesk Onyx HTTPS redirect .