Split Level - Floor Issue

Hi, I am trying to build a split level like this one sims-online.com/create-a-split-level-room-sims-4 Unfortunately, it doesn't seem to be working. I followed the instructions perfectly and watched many other tutorials on the subject but, when I try to put the floor down in the sunken part of the split level, it says "must be applied in a room". What am I doing wrong? What should I do?

I have a second floor, so the first floor has a ceiling, there is a roof... The room if fully closed with walls, there are windows on the walls and all... I don't understand :(