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);

As the error message says, you have probably forgotten to drag the transform of the playerBody into the public field in the inspector labeled PlayerBody.