What causes this error?
Airbnb checks your Uplisting calendar both before accepting a new booking from a guest and accepting any alteration requests. This check is very helpful as it means you don't get double bookings, however, it can mean you see an error like this.
The error is almost always (99%!) caused by a date being blocked throughout the duration of the booking length. If the booking alteration is from the 18th of May till the 25th of May, and there is an unavailability marker on the 19th then the alteration will fail and result in this error.
How to solve this error
Check to make sure all the dates through the duration of the proposed booking start and end dates are available on Uplisting. This includes unavailability markers, bookings (not including the booking being altered), blocked by booking window unavailability markers and any other unavailability markers Uplisting supports.
If there are any unavailability markers, remove them. At least temporarily. If there is a booking window rule blocker covering those dates, remove that rule, at least temporarily.
Once the dates are clear and open, you or your guest can accept the alteration.