If you try to save an appointment in the calendar but get a red "There are no rooms available at this time" error, it is because a room that can accommodate the service is not available for the entire duration of the appointment.
Calendar room view
See your room availability easily by viewing your calendar in room view.
Move appointment to another room
Click on the appointment
Use the "Room" dropdown in the left hand drawer to select a new room.
Click "Save"
If there are no other rooms in the dropdown to choose from, your service has not been set up to be performed in any other room. Edit your service in admin.