@Melissa802
If you (host) do not have a booked reservation from your end...... you need to let your guest know that you don't have a reservation for him. In this case, the guest should be the one to contact Airbnb to find out what the problem is. Even if you (host) contact Airbnb, it wouldn't get you far since you have no booking info, no reservation code to ask about.
Double check your "Reservations" page and your calendar to make sure. If you really do not have a booking for him, get screen shots for proof and send them to your guest. Make sure you keep all communication thru the ABB messenger. You won't be paid for booking that doesn't exist so make sure this is all sorted out before you let the guest in 🙂
Good luck~