pos is the mouse position, so it's a Vector2. A Vector2 doesn't have a
position property, so that's why you got an error.
I't snot clear what you're trying to do. You're not spawning any box. If you wanted to do that, you'd need to call
instance() on your
box PackedScene. In that case, you probably want to do
box.position = pos to set the box's position to the mouse position.