Standard building parts will snap to the foundations but not on the edges

Another user had an idea that I hope to capitalize on later this week. I'm looking into duplicating the fence foundation from vanilla Ark to a castle mod piece that can be placed basically anywhere, (including on my ceilings, etc), that will count as a foundation but also allow vanilla pieces to snap to it. Hopefully that will help to fill in the gap where my foundations are so big the vanilla pieces act like there isn't a foundation around.

btw, what does too high above ground mean, its not even high its like 1 floor high only? most of my walls give this error. how to fix it?

If you mean when placing a foundation it means that the game code failed, at that part of the map, to determine the correct ground height. For some reason it randomly takes the bottom of the world as the ground it measures on and it usually happens when the top of the foundation is too close to the actual ground.