This happens normally when your rate type is not assigned to that room type.
How to Access: In order to assign rate type to that room please follow the below steps.
User Privilege: eZee absolute configuration >> User >> Edit user >> Privileges >> Configuration >> Access to Configuration Panel.
Login to eZee Absolute Front Office >>> Edit the Reservation for which you want to assign the room and check the rate type of that reservation.
Location: Now, login to eZee Absolute Configuration and go to Rate type list >>> Edit the same rate type of that reservation and check if the desired room type is assigned to that rate type.
Once this is done, then edit that reservation and assign the room.