- Furnace is not processing
- Furnace contains an iron plate
- No window is open
Insert fuel, watch the ore processing to plates, and remove them immediately. Even once you close the window, #2 is now false so you can't proceed. The only way to continue is either putting the plates back in or smelting some more without making the same "mistake", neither of which are obvious.
Admittedly I'm not a new player, but taking the plates is a pretty obvious course of action. So either #3 needs to be removed so it always triggers before you have a chance to take them, or better yet, it could also check for the character inventory containing plates and skip to the next objective (craft a burner mining drill) if so.