In your function onTimertimeout(), you writed this :
enemy.position = randindex.
The question is that the type of position is Vector2, but the type of rand_index is int.
You can't assign an int variable to a vector2 variable.
Ok, you says you got null when you print(options).
I think that's because array can't contain node element.
You should use "String" with function get_node().
I mean you can do like this following:
var options = ["Position2D", "Position2D2", "Position2D3", "Position2D4"]
func _on_Timer_timeout() -> void:
enemy.position = get_node(options[rand_index]).position
(or you can use "global_position" to replace "position".)
This way in my test is fine, hope this help.