I'm not super confident in my godot abilities yet but I think I have ran into something like this. The problem for me was that the ballmodel node was above whatever node is creating a copy of it. This matters because the ready functions are called from the bottom to the top, so the ball model might not be ready when you are trying to duplicate it. If you can, move the ball model under this node. Maybe that's not happening here though, but I think the next paragraph will definitely fix it.
Something I feel like I should add, though, is that usually when adding a thing to your game, I think it's best to instance a scene. This can be done fairly easily by right clicking on your ball model in the scene tree and clicking "save branch as scene". Then when you want to add it, call preload("the path to your ball model scene").instantiate(). Then you can add that as a child like you are doing here.