You need to create a room where you can use for testing. Go to Test center and click Create room

Fill in the name of the room and the maximum number of candidates. It is optional to fill out How to find room.  Click Save room

The room is now visible in the list. The number of rooms available is shown with a digit next to the Rooms bar. You can edit a room by clicking at it and save the changes afterwards. You can also remove a room by clicking Remove room and confirming the action.