Health Reduction On Collision,Health decrease on Collision

I am not an expert in unity but I 've been trying to make an enemy health script which isn’t working properly. The number of ‘hits’ isn’t increasing at all even on collisions. the collisions happen and enemy is pushed away by bullets but the bullet hits do not increase. Please help.

public class EnemyScript_1 : MonoBehaviour
{

    public int hits = 0;
    public NavMeshAgent nav;
    public Animator anims;

    void Start()
    {
        nav.GetComponent<NavMeshAgent>();
        anims = GetComponent<Animator>();
        hits = 0;
    }

    void Update()
    {
        GameObject player = GameObject.FindGameObjectWithTag("Player");
        Vector3 playerpos = player.transform.position;
        nav.SetDestination(playerpos);

        Debug.Log(hits.ToString());

        if (hits >= 4)
        {
            Destroy(this.gameObject);
        }
    }

    void OnCollisionEnter(Collision other)
    {
        if (other.gameObject.tag == "Finish")
        {
            hits += 1;
        }
    }
}

first of all,

GameObject player = GameObject.FindGameObjectWithTag("Player");

shouls go in start, not in Update().

does the object that has the script on it also actually have an collider component?
maybe try

other.gameObject.CompareTag("Finish")

in the if statement