Well the server is 1.12.2
So that mode is the best to play.
There sone recent lagg issues making that you can only really play on 1.12.2 without disconnecting. These will get fixed asap.
You should be able to use 1.12 however 1.12.2 works the best. I don't know why it doesn't work on your computer and sadly can't help it
If 1.12.2 really doesn't work, just wait until the lagg issues are fixed. Otherwise tey to fix the 1.12.2
Good luck!