It would help if you provided a snippet of the problematic code. Identifier not found simply means you're using an unknown property be it variable or say key id.
As for keyboard input, WASD controls can be done like this
func _process(delta):
# vertical
if Input.is_key_pressed(KEY_W): # do action here
elif Input.is_key_pressed(KEY_S): # do action here
# horizontal
if Input.is_key_pressed(KEY_A): # do action here
elif Input.is_key_pressed(KEY_D): # do action here