That same thing happened to me. A guest put in a reservation request then realized they request one day less than they needed.  I thought declining so they could rebook would work. The days the guest wanted are unblocked so the guest can rebook. But it seems the Airbnb site will not let them rebook after we declined, even with the days unblocked. In hindsight, I think we should of accepted  the request for the day short and THEN altered the reservation to add the day. We are still not champions using  the Airbnb website!