@512af1fc5025402bHi, there
Has the problem been resolved?
I am playing on a PC but just tried it with some Sims who had just finished their graduation ceremony.
Immediately after graduating, they could not re-enroll.
The next day, Sunday, they could not re-enroll either.
They moved household on Sunday morning and was able to re-enroll on Monday.
Did your sim go to university from home? (My sim went from home).
If so, you may want to try moving once.
Hope this helps.