You should give more information with your posts :) like how your building the house. Did you use the house placement tool then click on the sign to build this?
 
I'm using the house placement tool and then customize. I tried using pandora but it never aligned properly. Sorry I didn't add more detail
 
With the gump open for building-look on the far right side where I point to the 2nd story. Each one of those will allow you to go above to the next
floor

-Image2.jpg
 
Go back down to the first floor- look for any thing which looks orange in color (error) all walls must be in place also check for any errors you might have for the floor
 
You can't make things fly unrealistically.
If you only have walls on the border of the foundation, and a floor covering the entire second floor, you must consider adding some support on the first floor (pillar, wall, etc).
 
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