enemies Scoring points killed

Hello, how can I make a score for each killed enemy? I tried to follow this guide: https://unity3d.com/learn/tutorials/projects/survival-shooter-tutorial/scoring-points

But can not I :(

In my code I also use the spawning of the enemies and it is the following:

Enemy script manager

public class Zombie : MonoBehaviour {

     protected float damage = 1.0f;
     protected float health = 1.0f;
     protected float velocityFactor = 1.0f;
 
     protected NavMeshAgent agent;
     protected ParticleSystem explosion;
 
     void Update(){
         agent.destination = GameManager.GetCharacter ().transform.position;
         if(!explosion.IsAlive() && explosion.gameObject.activeSelf){
             Death ();
         }
     }
 
     public void SetVelocity(float value){
         
         agent.speed = value;
     }
 
     public void SetDamage(float value){
         damage = value;
     }
 
     public void SetHealth(float value){
         health = value;
     }
 
     public float GetVelocity(){
         return agent.speed;
     }
 
     public float GetHealth(){
         return health;
     }
 
     public float GetDamage(){
         return damage;
     }
 
 
     void ApplyDamage(float damage)
     {
         if (health <= 0.0f) {
             StartExplosionParticle ();
         }
 
         health = health - 1;
 
     }
 
     void OnCollisionEnter (Collision coll)
     {
         if (coll.gameObject.tag == "Bullet") 
         {
             ApplyDamage (1.0f);
         }
         if (coll.gameObject.tag == "Bullet") 
         {
             ApplyDamage (1.0f);
         }
         if (coll.gameObject.tag == "Bullet") 
         {
             ApplyDamage (1.0f);
         }
     }
 
     private void Death(){
         Destroy (gameObject);
         GameManager.spooneEngine.GetCurrentWave ().Remove (gameObject);
 
     }
 
     private void StartExplosionParticle(){
         gameObject.GetComponent<MeshRenderer> ().enabled = false;
         gameObject.GetComponent<BoxCollider> ().enabled = false;
         explosion.gameObject.SetActive (true);
         explosion.Play ();
     }
 } 
 

And in every enemy he has this script:

 public class FatZombie : Zombie {
 
     void Awake(){
         explosion = transform.GetChild (0).GetComponent<ParticleSystem> ();
         agent = GetComponent<NavMeshAgent> ();
         health = 5.0f;
         damage = 2.0f;
         velocityFactor = 1.5f;
     }
 
     public float GetVelocityFactor(){
         return velocityFactor;
     }
 
 }

how can I do?