Easiest way to install or upgrade MC on a Raspberry Pi (or really almost any Linux distro) is by using the installJRMC script.
But if you want to know what the likely issue is (which I'm going to guess as I have no time to test it), I'm guessing the problem you're experiencing is related to the links in the commands - they should probably be changed from http:// to https:// since that changed a good while ago, as using https:// would be recommended in my mind. Also you *should* be able to install the MC30 latest repo and pull MC31 from it as long as you switch to the Bullseye branch. But, again, I can't stress enough how much easier it is to use installJRMC to install MC on most Linux distros at this point, basically one stop and there ya go.
But if you do it manually you likely need to edit the .list file and change whatever is used (Buster or Jesse) to Bullseye like Bob said in the release topic, as I mentioned above MC31 is in the Bullseye branch. I actually use the jriver.list file that the installJRMC script created (if going this route, delete the mediacenter.list files first) and what I did was I edited the jriver.list file in a text editor and changed it to Bullseye and it works great for me for MC31 on my Raspberry Pi. IMO there's really no need to have multiple mediacenter.list files anymore, just make a generic one and change it when a rebase happens (e.g. Buster or Jesse to Bullseye).
EDIT: Seems like changing http:// to https:// doesn't work at all... that's not good IMO, hopefully that's something that can be fixed and supported in the future...
