From your error message it seems as though you might have Debian 7 repos still enabled.
Confirm which version you are actually using by running:
cat /etc/debian_version
Now, no matter if you are running 7 or 8, you really must upgrade as soon as possible. Debian 8 was EOL last summer meaning that you haven’t had security updates for your server in many many months. Needless to say that’s no good for a public facing server!
Virtualmin will never update your OS, this you need to do yourself by editing your sources.list and running the upgrade procedure. If you Google “upgrade debian 8 to 9” you will find tons of easy guides from well known linux sites.
Mostly Debian upgrades are fairly problem free but in any case make sure your backups are working and prepare yourself (and clients) for some down time while the upgrade is running.
If you’re running 8, you will need to first go to 9, then onward to 10.
If you by any chance are still on 7 you might be in for a nasty surprise as most of the Debian 8 repos will be disabled by now.