Alright, calm down now, this isn't the end of the world.
There are a couple things that can cause this to happen. First of which is that you've closed out of the build menu by accident. Make sure you're clicking the big "CONFIRM" button to the instead of the smaller icon to the right of it with the nail and hammer crossing. It's nothing to be ashamed of; I did this all the time when I got started.
Second is just a plain old glitch that I've seen through my hours of playing, where it does indeed show the building as green, but not allow it to be placed in the editor. Exit the editor, quicksave, close the game, boot it back up, try again; it's unlikely you're going to get the same problem twice in a row.
No,no message error,the building is green, i left click and nothings happens.Right click works, and the rest of functions too.I have no problem with my mouse of another hardware,but when i enter in buiding mode,neither a wall nor a simple bonfire can be constructed, in my outpost or any place.
In fact,I left click and the building simply dont place.But it is all resolved.I had changed the key controls in my game,for confort,i return this to Predeterminated and i t get fixed.Thanks all for your time and sorry for the bad english xd