You can not add an extra slot if you already have 12 slots on a server.
Here's a quote from the patch notes saying subscribers are LIMITED TO 12 CHARACTERS PER SERVER
Character slot restrictions are now being enforced:
By default, Free-to-Play players are restricted to 2 characters per server.
By default, Preferred Status players are restricted to 6 characters per server.
By default, Subscribers are limited to 12 Characters per servers.
All players have a Global Active Character Limit of 350 Characters.