To add to what Ilia said: If you aren’t technically very proficient, please don’t try to install on an unsupported distro and use it in production. If you just want to use Virtualmin, install it on a supported OS.
Dev versions of the installer should only be tackled by people who want to help develop…you should be comfortable filing (good) issues on github, poking around in shell scripts, dealing with dependency issues in your package manager, troubleshooting why something failed so you can post your solution rather than your questions (don’t ask for support with an unsupported distro in the forums or otherwise…unsupported means unsupported!). Not trying to discourage people from giving it a try, but also please be cognizant of the fact that any time we spend supporting folks with questions about Debian 11 is time we can’t spend on finishing the installer. If you find bugs and fix/report them, we’re happy. If you just ask us to help you solve problems as though it were finished, you’re taking time away from the work.
Also be aware that there are a least two or three pretty major changes happening in the installer (different packages and some config changes)…so you’re going to have a half-6-half-7 hybrid installation. We can’t support that properly, either, even once everything is done. Plan to reinstall when version 7 comes out. Debian 11 will never be supported on a Virtualmin 6 installation, and the installer today is mostly still a Virtualmin 6 installation (with a few minor changes)…so you’re getting a unsupportable installation. If you’re technical, go nuts. If you just want to be a Virtualmin user, pick a supported OS or wait until Virtualmin 7 is announced.
Also, please stop asking when it’s coming. We are not going to pre-announce, because it’s always wrong when we do.