OnCollisionEnter is not working...

OnCollisionEnter event is not working when camera (FPS Character Control / Capsule Collider) collides an object

99.9% of the cases, you missed out the Rigidbody requirement.

Straight from the docs:

Note that collision events are only sent if one of the colliders also has a non-kinematic rigid body attached.

I have solved this problem myself by creating an external capsule collider (A transparent capsual having no rigidbody containing mesh collider).

It is working properly