Strange issue today while I’m trying to migrate a VPS from Ubuntu 16.04 to 20.04. Virtualmin is working fine, PHP-FPM is available as an option while I create a new server. But as soon as I restore the backups from 16.04 the PHP-FPM option is not even showing and I can’t enable it for the restored servers.
There are config files per virtual server here: /etc/webmin/virtual-server/domains/ and named as a long number.
When migrating to another server or restoring on a fresh (and newer) installed OS, you probably have a updated PHP version. Look in these files for the line beginning with php_fpm_version.
In my case the fresh install was equipped with PHP 7.4 while the old system was running PHP 7.2.
The restored vserver had in it’s config file php_fpm_version=7.2 which was not found on the new system. I corrected it to php_fpm_version=7.4, saved it and went to Virtualmin > Website Options and the option to select PHP-FPM appeared.
I noticed something similar months ago on CentOS 8 after installing an extra PHP version. PHP-FPM pool files weren’t adjusting to the new version but moving and editing them as @brainf mentioned fixed it.