Error "NullReferenceException: Object reference not set to an instance of an object". The object is called, so what could be the error?

Question

I’m having this problem for a while and can’t solve it: I keep getting this error at line 20 and 38, and the health_Bar functions keep failing.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Enemy : MonoBehaviour
{
    Animator animator;
    float maxhealth = 20;
    float health = 20;
    [SerializeField] private Health_Bar health_Bar;
    [SerializeField] float speed;
    [SerializeField] float range;
    [SerializeField] float maxdist;
    Vector2 waypoint;
    SpriteRenderer spriteRenderer;

    public void Health(float damage) {

        health -= damage;
        health_Bar.UpdateHealth(health, maxhealth);
        if (health <= 0) {
            Defeated();
        }
    }

    public void Defeated() {
        animator.SetTrigger("defeat");
    }

    public void Removeenemy() {
        Destroy(gameObject);
    }

    // Start is called before the first frame update
    void Start()
    {
        animator = GetComponent<Animator>();
        health_Bar = GetComponentInChildren<Health_Bar>();
        SetNewDestination();
        spriteRenderer = GetComponent<SpriteRenderer>();
        health_Bar.UpdateHealth(health, maxhealth);
    }

As you can see, the problem is with "health_Bar.something". I can’t figure out this problem and tried everything I can imagine and kept getting the same error.

What is the explanation for this error?