@Lionel64 This a forum for hosts and guests to exchange information and questions. You'll have to contact airbnb to ask for a cancellation. If the second floor/stairs situation wasn't mentioned anywhere in the listing description, airbnb might refund you, under the extenuating circumstances clause. But if you have health or mobility issues, it's really important for you in the future, if using airbnb, or any online booking platform, to send an Inquiry to the host before booking anything, making your issue clear, and asking enough questions to ensure that the place won't present any problems for you healthwise.
You can't just assume that there won't be any stairs and then cancel because there are- it's really not fair to the host who has cleaned and prepared the place for you and won't be able to rebook it on such short notice. When you cancel on arrival, the host has lost their earnings for that day. It's not like a hotel, where if you decide not to take the room, it doesn't matter because someone else will probably walk up to the front desk and take that room within a few hours.