Is it a world that you uploaded or is it just the default world? By default the world is called world, so the command would be "/dl world world" without the quotes. You might also be getting that error because the world has been deleted. If someone ran /delete world (worldname) I think that's what trying to download it would look like, idk. I don't remember if you're able to download a world from your server console or not, someone else may have to clarify that for me.