If you have followed the Un-Cancel Non-Session Bookings Knowledge Base article, but are receiving the below error message, this is because you are trying to un-cancel a session based booking.
Sessions have strict availability, and when cancelling this booking, the availability on that booking gets released, so that this can be booked by another customer. Therefore, you cannot re-allocate the session availability, as this could cause double-booking seats and overbooking on the session that would since have further bookings on it.