Help? "Incompatible with Terrain Constraint TerrainTile"

I was building my backyard, trimming my pool to make room for a rocket ship and everything was fine. I exit build mode and go to send my sims upstairs to sleep but the stairs disappeared. I thought I accidentally deleted them and when I went to replace them (in the exact same spot) this happened. I hope there is a fix soon because I have been looking for hours.