Regarding obtaining a Let’s Encrypt certificate for Webmin to use, it failed for me the first time.
After a fresh install of CentOS 8 (and running the install script, re-running the script after the “PID” error and fixing the mySQL root password) I headed to Webmin > Webmin Configuration > SSL Encryption and the Let’s Encrypt tab.
The first time the Website root directory for validation file was not set so I selected Other directory" which i thought contained /var/www/html. The request failed.
Then I looked more carefully and saw some fuzziness around the ends of the path. In fact it was (double) quotes.
Removing the quotes so that the Other directory was /var/www/html resulted in a successful request and installation of the LE certificate.