I'm having this issue as well. Stuck on the 3/4 counter.
Spaninq helpfully pointed out that the de jure borders of Serbia might change if you sleep too long on it, like I did.
I have a feeling this was my issue. I loaded back up into my game with a screenshot of Serbia's borders at the beginning of the challenge, but ran out of time before I could fabricate claims
Maybe I'll just do a second run and remain a part of Hungary while solely focusing on Serbia. This was a fun challenge though for casuals like me since I never really start games outside of 1066 and earlier bookmarks.