Elton:
Same thing happened to mine. I don't know if the prior owner EVER took off that seat, but it was stuck on like glue.
Pushing down in the middle didn't do it, and I turned that key so hard that I thought I was going to break it.
Eventually, I hosed the locking mechanism LIBERALLY with WD-40/silicone and then took the lubricant can with a long straw and literally doused the underside of the seat in an attempt to lube up the seat latch.
Then I wiggled the key like crazy whilst pushing down on the rear-center of the drivers seat and finally it came free.
Then I hosed the living hell out of everything related to the lock with lubricant to ensure that this little event did not repeat itself.
It's such an annoying problem. Good luck!