@Sean1066 I just tried booking your place for the earliest available dates (mid-February) and I didn't have any issues selecting the non-refundable option. Are they trying to book for tonight or something? That could be an issue.
If the guest is having issues with booking, I would refer them back to Airbnb customer service. If there is a problem with the site or their account, there is nothing you can do about it and you're just going to waste time and get frustrated. You pay Airbnb to handle the bookings: let them assist the guest.