I plowed a large flat area of land that no one owns, far away from anything really, but when i try to build a cabin on top, it says i cannot build there?
I guess you have to pave the area. You probably could have built the cabin on unplowed ground though. Building things on paved ground makes them decay slower, but since cabins don't decay (yet) it probably doesn't really matter.
Take my advice with a grain of salt though, since I haven't build my own cabin since the reset and I take my knowledge from dev announcements.