Just before I started building any houses/buildings on my lot, I ran into that error = Can't build outside of unit.
Not sure why that happened, so I just went back to a previous save.
However, if you don't have a previous save or don't want to use it. You might try:
RestrictBuildBuyInBuildings false (Turns it on)
RestrictBuildBuyInBuildings true (Turns it off)