You’ll need to figure out where the commands are, and update the Module Configuration (click the gear icon 
in the upper left part of the content pane in the Mailman module).
I’m surprised it’s moved yet again. It seems like we updated the location for Debian 11. That’s annoying. virtualmin-mailman/config-debian-linux-11-ALL at master · virtualmin/virtualmin-mailman · GitHub
You can use your package manager to find where things are: dpkg -L mailman3