Hej! I have the same issue. I guess a bad interaction between the Terrain and the fence Frame is causing an error, so the game doesn't allow us to build (a little Underground)
Same Problem occures with plants reaching into the fence construction site, like bushes.
It is Pretty annoying to build up a base Right now. only every 3rd or fourth fence will be build. And this in almost flat Terrain. Pleeeeese fix!