The specific solution for the error you're getting—the error that the mirror you're using no longer exists—is in the PSA:
xenopeek wrote: ⤴Thu Jul 22, 2021 1:33 am
What if you are affected?
If you are affected simply click on the main mirror in Software Sources and choose a different mirror from the list. That fixes this problem.
Afterwards your Software Manager and Update Manager (have you not been installing updates?) should work fine again.
As for Signal, that is not available from the official software repositories. You can either install it from Software Manager as a flatpak—which despite the comments runs fine but will run the software in a security sandbox to protect your OS from internet nasties and that may limit if you want to download stuff from it, though you can easily reconfigure this with Flatseal flatpak—or you can install it from Signal's own repository, that they spell out how to add on their download page:
https://signal.org/download/