Health and Damage [C#]

Your bullet script is not attached to the bullet your are firing. In

 shot = Instantiate(bullet, spawnpoint.position, transform.rotation)as Rigidbody;

You make a bullet and apply force to it. But the OnCollisionEnter() is not working as the shot you made doesn't have the bullet script

You should move your

 if( canControl == true ) {
    ...
 }

to another Firing script. What you will have then is

a) Damage Script (Attached to an enemy)

b) Bullet Script (Attached to the Rigidbody bullet)

 public class Bullet : MonoBehaviour {
    public float damage = 10.0f;
 
    void OnCollisionEnter(Collision col)
    {
       col.gameObject.BroadcastMessage("ApplyDamage", damage); 
    }
 }

c) Firing Script (Attached to an empty object or your player)

 public class Firing : MonoBehavior {
    public Bullet bullet;
    public float Force = 2000.0f;
    public AudioSource Shoot;
    public Transform spawnpoint;
 
    void Update () {
       //Nothing's changed from your previous code
       if(canControl == true){
       ...
       }
    }
 }

Friendly Tips: Check the setup of your bullet and enemies ( Collider & Rigidbody ), then use a Debug.Log("Collision with " + col.gameObject.name) to check whether a collision has occured.

Lastly, this is not related to the question but

 public void ApplyDamage (float damage)
 {
    Health =- damage;
 
    if (Health <= 0.0)   // <---What does this one do? Are you planning to call Detonatin() here?
       return;
 }

I tried to add a rigidbody when the enemy is dead, so that it is not moving anymore. But it does not really works.

 using UnityEngine;
 using System.Collections;
 
 public class Damage : MonoBehaviour {
 
     public float Health = 100.0f;
     public Rigidbody deadReplacement;
 //    public Rigidbody dead;
     public bool deadTrue = false;
     public GameObject gameObject;
 
     
     public void ApplyDamage (float damage)
     {
         Health =- damage;
         
         if (Health <= 0.0){
         deadTrue = true;
         }
     }
     
 //    void OnTriggerEnter(Collider bullet)
 //    {
 //        GetComponent<Bullet>();
 //        Health =- Bullet.damage;
 //        
 //        
 //    }
 
     
     
     // Use this for initialization
     void Start () {
         
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     public void Detonatin(){
         
         Destroy(gameObject);
         
         // If we have a dead barrel then replace ourselves with it!
     if (deadTrue = true){
         Destroy(gameObject);
         Rigidbody dead = (Rigidbody)Instantiate(deadReplacement, transform.position, transform.rotation);
 
         // For better effect we assign the same velocity to the exploded barrel
         dead.rigidbody.velocity = rigidbody.velocity;
         dead.angularVelocity = rigidbody.angularVelocity;
             
     }
         
     }
 }