OnTriggerStay triggers when any game object with a collider (on one of the physics layers which collide with that of this object) collides with the object. You might be registering a collision with something other than the player. So you should check if the object you are colliding with is actually the player. A good way to do this is to set the tag of your player object to "Player" and then check for that tag in the collision method:
void OnTriggerStay(Collider other)
if (other.gameObject.tag == "Player")
other.attachedRigidbody.AddForce(Vector3.up * 100, ForceMode.Impulse);
When you still get that error instead of the expected results, then the most likely explanation is that you think you added a rigidbody to your player, but you actually didn't. In that case please edit the question with some screenshots which show how your player object looks in the inspector.