This isn't my post, pulled from a Reddit post however I'm in the same boat:
One of the 3 Hogwarts secrets involves using an item called “key of admittance.” I won’t explain where it is due to spoilers but you first gain access to the location during a main story mission.
I found out the hard way that if you try to use the key during that mission you won’t be able to open the door because the “return to mission” timer will hit zero before the door opens. Unfortunately, on reload it seems as though the key has been used up and the door will never open. I didn’t notice until it was too late and my only useable save was way too old to go back… I’m locked out of 100 % which is pretty disappointing. Hoping they’ll fix this glitch soon but I dont see anything about this online so I don’t have much hope.