One thing I've found is if you are trying to place stairs on the ground level you need to put down floor tiles first. For example, my game never lets me place stairs down from the second floor onto the grass if it's inside a room, I have to cover the grass with floor tiles first.
ThornQueen, if you could provide a more detailed description or a picture of what you're trying to do, then we could give you specific help and advice.
Have you tried using the ground leveling tool to level the ground around where you are trying to place the stairs? Try placing floor tiles. I've never encountered "terrain too high" but it sounds like you need to lower the ground a bit more. Sorry if you've tried these tricks already. A picture would help us see exactly what you are trying to do.