Assets/health.cs(11,23): error CS1519: Unexpected symbol `=' in class, struct, or interface member declaration

@c4k3m45t3r These seems to work for me not much was changed compared to your code.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 public class Health : MonoBehaviour
 {
     [Header("References")]
     public Slider healthbarslider;
     public Text gameOverText;
 
     // Use this for initialization
     void Start()
     {
         gameOverText.enabled = false;
         healthbarslider.GetComponent<Slider>();
     }
 
     void OnCollisionEnter(Collision col)
     {
         if (col.gameObject.name == "object")
         {
             healthbarslider.value -= 10f;
         }
         if (healthbarslider.value <= 0)
         {
             gameOverText.enabled = true;
             //System.Threading.Thread.Sleep(5000);
             // Will load level 0 change this to ur level int id u need.
             SceneManager.LoadSceneAsync(0);
         }
     }
 }