It means Godot can't find the animation tree, probably because it's in another path.
So look here:
statemachine = $AnimationTree.get("parameters/playback")
That would only work if your animation tree is named "AnimationTree" and is a child of the current object attached to the script. If it's somewhere else, or named something else, then you would need to edit that line. For example, if the object is a sibling to the current script (on the same level) you could do:
statemachine = get_node("../AnimationTree").get("parameters/playback")
If it's somewhere far away you can also use an absolute path:
statemachine = get_node("/root/Game/Player/AnimationTree").get("parameters/playback")
In this case I made up "Game" and "Player" so you'd have to edit that ("Game" would be the root node).