[uMod] 03:07 AM [Error] No resource available to update for package games/rust (pre-release: no)
[uMod] 03:07 AM [Error] Core install failure

?

Is there a particular reason you are trying to install uMod instead of Oxide? uMod isn't released, so we generally wouldn't recommend using it at the moment aside from testing.

To answer your question though, there are ky pre-release packages available, so you would need to use the -P parameter when installing and updating.