i dont know what the error message is, but i just try by guessing it,
if the error is in this line:
var b = Bullet.instance() <------- error
maybe there are some possible problem, if the indentation is not the problem, and i dont know how "relative new" you are, but are you just write this line:
export (PackedScene) var Bullet
without load the scene from the inspector?, sometime i forgot about this too, and im also relative new to godot, hope this can be help, im sorry if im wrong