There would need to be a small space between the interior walls and exterior walls, so they are not touching. There cannot be any overlapping walls on the same building location. If they need to overlap, you can insert a new building location by going to: settings>building locations>add location
It looks like the framing mode may be turned on. You can turn off framing mode by going to:
view>framing>display all but framing or view>framing mode on/off