Building a room and half way through when i attempt to place a wall i get a yellow x instead... not really an x but a three axis yellow marker... what is happening?

Try looking UP. Wall pieces that are "meant" to be higher up the wall have their axis below the object instead of on top of it, supposedly to help alignment, but I find it annoying. So scroll back and look up, sometimes WAY up, depending on the piece. If that's not the problem, there might be some other ideas.