Weird question: why is this in the physics process? Do you need this triggering every physics step?
Seems like input would be a better place... Not sure if that solves your problem.
EDIT: Also adding Ramazan's fix, which looks right (and often is).
onready var sandScene = load("res://Scenes/Sand.tscn")
onready var sand = sandScene.instance()
var s = sandScene.instance()
s.position = Vector2(rand_range(10, 980), rand_range(10, 980))