I tried to apply the head rotation and this problem is seen

UnassignedReferenceException: The variable playerBody of MouseLook has not been assigned. You probably need to assign the playerBody variable of the MouseLook script in the inspector. UnityEngine.Transform.get_localRotation () (at :0) UnityEngine.Transform.Rotate (UnityEngine.Vector3 eulers, UnityEngine.Space relativeTo) (at :0) UnityEngine.Transform.Rotate (UnityEngine.Vector3 eulers) (at :0) MouseLook.Update () (at Assets/MouseLook.cs:26)

// Code starts from here

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class MouseLook : MonoBehaviour
 {
     public float mouseSensitivity = 100f;
     public Transform playerBody;
     float xRotation = 0f;
     // Start is called before the first frame update
     void Start()
     {
         Cursor.lockState = CursorLockMode.Locked;
     }
 
     // Update is called once per frame
     void Update()
     {
         float mouseX = Input.GetAxis("Mouse X") * mouseSensitivity * Time.deltaTime;
         float mouseY = Input.GetAxis("Mouse Y") * mouseSensitivity * Time.deltaTime;
 
         xRotation -= mouseY;
         xRotation = Mathf.Clamp(xRotation, -90f, -90f);
 
         transform.localRotation = Quaternion.Euler(xRotation, 0f, 0f); 
         playerBody.Rotate(Vector3.up * mouseX);
     
                
     }
 }