A slight educated guess. The get node command isn't finding your Player node.
Put a print statement and if it's
null then that's what it is:
What I do is use a Global script to set the player node in the player ready function:
Global.player = self
then you can access the variables in you timer function like this:
Global.player.ACCELERATION = 10
You will have to make sure the player node sets its node value before accessing it with other scripts. Either spawn your player first or check that
Global.player is set.