The repository only allows downloading/updating to public releases (latest public release is 1.19.5.3112).
If you want to install a beta version (latest public beta is 1.20.0.3181) you need to manually install it, e.g. using dpkg.
Edit: if you want to use public versions and not be informed about beta updates, you can change the update channel in Settings > [Server Name] > General > Server update Channel to Public
