right so ive made a slope which is 0.5 in difference from the tile before and in observe its showing up as green but trying to build a wall on it fails just before the action finishes, is this a bug or can we not build on slopes?
For me it took 5 attempts on some items ... at some random times ... to complete. And it sounds like the exact same issue you were having ... maybe just keep trying and see if it will succeed ...