Really sorry, it's too long ago to remember the exact error,
but IIRC
when you followed the ubuntu instructions for adding the repository it would kick an error because the command included a reference to noble and mint os name is xia so the contents of the osrelease when checked didnt match and it threw an error.
Could be wrong, I didnt document it.
The work around was to edit the commands.
All a moot point now as
a) the instructions now on the mullvad site don't reference noble and
~~b) mullvad now appears to be in the mint store (which is how you should always install if possible~~
Not correct, only true if you've manually added the repo
Ok we're talking at cross purposes.
Yes, mullvad has a noble repo, it doesnt have a xia repo (which is the mint equivalent version name)
Attempting to add the mullvad repo using the old ubuntu instructions failed because noble =/= xia
Yes you can work around that but its not beginner level.
Op asked if Flatpaks was the answer whivh imho it isnt, the best answer is downloading the .deb