Back to the Wall challenge???

I'm playing Sims Freeplay on my phone, and I'm stuck at 82 percent on the " Back to the wall challenge"  I can't build a upstairs room, I've tried everything?  Been like this for 4 months. 

I even tried building a small room separate from the house like a garage & putting up wallpaper in there.   It's so frustrating 😫   Any help appreciated!!! 

Thank you!