If you previously have had a tenant profile to search for rooms with, but now have a room to rent out, you must change your profile status to 'Owner' before you can create an ad for the room.
You do this in the top of your settings in the first field called 'I am'. Choose 'Room owner' instead of 'tenant'.
When you have done this, your profile will change to landlord and you will now be able to make the ad for the room further down in settings under 'apartment information'.
Is it the other way around - you have a landlord profile but need to change it to tenant, the process is the same. Just choose 'tenant' instead of 'room owner'.