I just set up a test environment on a Debian 9 with Webmin/Virtualmin both latest versions. I created a virtual server, users, database and so on.
I Re-Checked the configuration and it says "... your system is ready for use by Virtualmin". In "PHP Versions" section it says "Your system only has PHP version 7.0.33 installed, so selecting a different version for each directory is not possible."
I was able to install phpMyAdmin 4.9.5 because PHP 7.1 is not available on this system from Debian's repository and is working as expected.
I wanted to install RoundCube and both versions in the list 1.4.8 (Latest) and 1.2.12 (LTS) refused to install. I got this error message: "Failed to install script: Could not find any values in section PHP". Screenshot attached.