Googling the dpkg error indicates it can happen if sendmail was previously installed. That wouldn’t be the default on a fresh Debian or Ubuntu system, so it would indicate some non-standard install process (or just not starting with a freshly installed OS, where stuff was already preinstalled). Or some other thing that led to sendmail being installed, and then removed to reinstall Postfix (which is our default mail server and what is installed when you run the Virtualmin installer).
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=939885
So, I think we’ll just say, “Don’t do that.”