First of all, that’s not the complete error message. In future, please include the full error.
“null instance” is what you get when you try to get_node() a node that doesn’t exist. That means that get_parent().get_node("Player") is not a valid node path. Check your scene tree again and use the correct path from the enemy to the player node.
This is happening because get_parent().get_node("Player") is returning null because Godot can’t find a node at that path with the name Player at the time it searches for it. It could be that your Player is being queue_free()'d while the game is running or there simply doesn’t exist a Player node at the path you gave it.
func _physics_process(delta):
var Player = getparent().getnode("Player")
if is_instance_valid(Player):
position += (Player.position - position)/50
look_at(Player.position)
move_and_collide(motion)
That code will eliminate the crashes but may not give you the behavior you desire. Your code will only work if your node structure is like this: