generally speaking 95% of the time you will have no problem buying barca tickets upto kick off time
rarely a complete sell out
pretty sure he means the tickets for the plane.
just call the company you currently booked with and change flight. because you're flying to spain and they have this weird passenger info rule you might need to book earlier than a couple of hours.
also -- landing at 18:50 -- assuming you're landing Girona -- means you'd have a sweat of getting to the game for kick off, fwiw -- takes about an hour in the car, so if you plane is early by just a smidge (and you only have hand luggage..)