As things are right now, only your player can detect collisions. For this to happen your player must have a collider attached, and be sure the IsTrigger checkbox is unchecked. If you want to make it react to IsTrigger than you should change your OnCollisionEnter() to OnTriggerEnter().
Collisions can only be detected if at least one of the colliding objects has a call to OnCollisionEnter() or OnTriggerEnter(), depending on if you want the collider to react to collisions or triggers, and obviously both colliding objects must have a collider attached.
For further detail on understanding how to properly handle collision detection among your objects, please see this question.
