Actually, "not possible" also happens when Airbnb discovers something concerning about a guest. I once rented around them when a young woman tried to book for herself and friends and suddenly it was "not possible." She had such an unusual name I looked her up on FB and discovered we had a friend in common. She was perfectly lovely and left the place spotless.
As typical, Customer Service called two weeks later about my inquiry and said "due to policy they couldn't say why they blocked her." I said, "I hosted her, you didn't get the fee, and you don't get to decide who I will or will not rent to without justification."
This is the same company that didn't screen out the known felon when he booked an home-stay with an elderly resident.
So depends. Could be the guest cancelled. Could be there was something Airbnb found (and they won't ever tell you what it is. I tried.)