Check the Virtualmin Dashboard to see which version of Webmin is running on your system. Is it the latest one, released today, and announced by Joe in the mesage above? I.e. Webmin 2.013?
If not, you have the earlier version of Webmin and there is a known bug in that version that causes the error you describe.
Peruse recent messages in the forum for more information about it.
And begin a new topic, let’s not elongate this one which is essentially an announcement.
You have to specify which repos you’re using. As far as I know, it has been pushed to all Debian repos that are maintained (i.e. /vm/6 and /vm/7), but if that’s not the case, I have a bug somewhere in my scripts.
$ cat /etc/apt/sources.list.d/virtualmin.list
deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-buster main
deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-universal main
$ apt-get install -s webmin
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be upgraded:
webmin
1 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Inst webmin [2.001] (2.011 . virtualmin-universal:virtualmin-universal [all])
Conf webmin (2.011 . virtualmin-universal:virtualmin-universal [all])
On a very recently installed Debian 11:
$ cat /etc/apt/sources.list.d/virtualmin.list
deb [signed-by=/usr/share/keyrings/debian-virtualmin-7.gpg] https://software.virtualmin.com/vm/7/gpl/apt virtualmin main
$ apt-get install -s webmin
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages will be upgraded:
webmin
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Inst webmin [2.001] (2.011 . virtualmin:virtualmin [all])
Conf webmin (2.011 . virtualmin:virtualmin [all])