First, make sure you have all the previous Keys. Then, talk to GPB in the Void Room and try again. If it still doesn't spawn, you've already collected it.
It's probably fake. Click here for a list of fake Keys.
See below or click here.
Just keep trying, you’ll get it eventually. Also, be sure to leave the room and come back after talking to him 6 times in a row.
Simply P rank The Crumbling Tower of Pizza.
Enter the doorway next to the elevator on any floor.