When I try to place a wall or roof on top of another wall, it does not allow to do it. How can I fix it?

Thanks, Sam The Wox

Are you using a structure made by someone else? Or is it something you made? Or, is it an in game piece you are trying to build with?

An in-game build piece.

Make sure your pieces are lined up. Then press shift and hold the mouse button up. I will try and make a video of it to show you what I mean if you are still confused. A side note: Sometimes I have similar issues when a piece is kind of blocking another. Sometimes it depends on where the piece is lined up on the grid. If you turn the grid on, it may help when you are building in the game.