It was all right, but I was looking in the wrong place for the free slots. I was trying to create the new domain by the "get DNS for an existing domain " option in the center of the screen, but the correct option was to have used the link from the top of the screen (You have 2 free slots left. Add Free DNS Domain you have already registered). I only noticed this link in the last days.
I'm sorry for the inconvenience and thank you for your support.