@Kevon9 . Hi Kevon, has the stay commenced? You cant charge extra fees once a booking has been accepted if it wasnt included in your listing unless guest consents. Late check out fees arent a separate item in the booking (but yes you could use the extra guest charges field as a once off).
If they have started their stay and now asked for a late check out alternatively you can use the Resolution Centre to request the fee from Airbnb; but you run the risk the guest wont pay for it. You are within your rights to decline a late check out request (past your stated check out time) but consider impact.
Sometimes its best to say you will try but cant guarantee a late check out. Otherwise if you want to do this regularly and charge a fee, include the info in your listing.