Ideas
I am having an issue where graduation doesn't trigger, but further along in the process. I can see it on the calendar and my sim does get the pop up to travel to it. Once there, there is no option on the building to "attend graduation" as expected after a time. Other sims in caps and gowns appear, attend ceremony and reappear to throw their caps as expected. My sim cannot participate in any of this. I've had it occur at both Britechester and Foxbury graduations (with different sims in this save file.) In both worlds I replaced the default lot your sim travels to with Residential Rental lot type; the commons is now at a different location.
Once I realized this was occurring, I replaced the original Ubrite commons lot back to that lot type and revisited my graduating household. The option to attend graduation is back on the building as expected. Could others have changed the default lot type to cause an issue with graduation occurring as expected?
I used mods, but removed and repaired before testing and reporting.
I hadn't changed any lot types, so I don't think this is the cause, for me at least!