This is because, with a FREE account, you can only have 3 floors and 3 designs. You should first create a new floor to have more designs. Click the “Settings” icon (3rd left icon in the top bar) to get a menu where you can add floors.
You can also upgrade to a PLUS account to have unlimited designs per floor.

kind regards,

Nico
Floorplanner Team

Is there a way to duplicate the outline of the main floor so that you dont have to go to all the trouble of creating the thing from scratch when it comes to the other floors in your house?