If you get an error when trying to book an appointment, check that your service is set up correctly.
But it could also be that the room that you are trying to book is just not free to the time you need to book the service. In the calendar, click on "Staff view" in the top right corner and select "Room view". This will give an easier view of whether there is a suitable room free.
"Room unavailable" or "Appointment requires a room" errors
The service you're trying to book may not have the desired room assigned.
Go to Admin/Location/Services and click on the relevant category or service
Click on "Rooms & Equipment" and tick all rooms that can be used for the service and in which priority order (ie which room should be selected first if free and so on).
Scroll to the bottom and save
Read here to solve Equipment isn't available in this room error