On Lemal, in place of a forge, there is nothing on that spot and you can build on that space.
Creeper can get to and destroy the forge. You have to get weapons there soon enough to protect it. Or the Fprge could be destroyed manually.
I guess one of those things might have happened. Try to restart the world and see if that helps. If not, then take a save right after you start the world, take a screenshot and post them both to this thread.