A PITA is a Pain In The Ass, also known as a big problem, something that is very hard or annoying to do, something that pisses you off, etc.
UrbanDictionary has this to say about it:
pita:
1) Acronym for Pain In The Ass, a major annoyance.
2) Flat bread of Mediterranean origin, eaten as is or filled with small pieces of roasted meat, veggies, condiments, etc.
1) Reinstalling everything after a virus was a true PITA.
2) Greek-style pita is round and thickish, while Cypriot-style pita is elongated and thin.
We’re not talking about the Greek one here, though.