This is happening when the room inventory is zero for that date on which we are looking to amend the stay or making a new reservation.
Solution: There are certain confirmed bookings for the same date, to which rooms are not assigned and you are trying to assign a room to NEW RESERVATION so the system will not allow doing that because you need to allot rooms to previous bookings means you have a reservation but not assigned room yet due to which we can see the reservation sells as empty and when we assigned them we can see the Filled sell for a particular booking.
In the stay view, you will find YELLOW colored header following the Room Type and it will display certain numbers (as shown in below screenshot) which specify the no. of bookings to which rooms are not assigned.
Solution: Thereby, to meet the conclusion, you will need to take an action (Cancel/assign room) on the previous booking, and only then you will be able to assign a room to this new reservation, If we cancel a booking then the inventory will rise and if we assigned a booking we can see the filled sell for that booking on Stay view. To take necessary action, you may click on YELLOW header showing numbers or you can go to reservation list and filter booking by UNASSIGNED ROOMS (Refer below screenshot). Now, here is the list of bookings to which rooms are not assigned. Thus, take the necessary action and you will be able to make a new reservation and assign rooms.