mouse look not working in first person controller script

Hello! I want to make a first person controller for my project, so I searched some tutorials and found a sample of code. However when I try and run it, it outputs a UnassignedReferenceException error, saying that the playerCamera variable has not been assigned. I already went through the code and I don't know how to assign it, and if I don't, the mouse look won't work and the player won't be able to look around. here is the script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [RequireComponent(typeof(CharacterController))]
 
 public class SC_FPSController : MonoBehaviour
 {
     public float walkingSpeed = 7.5f;
     public float runningSpeed = 11.5f;
     public float jumpSpeed = 8.0f;
     public float gravity = 20.0f;
     public Camera playerCamera;
     public float lookSpeed = 2.0f;
     public float lookXLimit = 45.0f;
 
     CharacterController characterController;
     Vector3 moveDirection = Vector3.zero;
     float rotationX = 0;
 
     [HideInInspector]
     public bool canMove = true;
 
     void Start()
     {
         characterController = GetComponent<CharacterController>();
 
         // Lock cursor
         Cursor.lockState = CursorLockMode.Locked;
         Cursor.visible = false;
     }
 
     void Update()
     {
         // player is grounded, so recalculate move direction based on axes
         Vector3 forward = transform.TransformDirection(Vector3.forward);
         Vector3 right = transform.TransformDirection(Vector3.right);
         // Press Left Shift to run
         bool isRunning = Input.GetKey(KeyCode.LeftShift);
         float curSpeedX = canMove ? (isRunning ? runningSpeed : walkingSpeed) * Input.GetAxis("Vertical") : 0;
         float curSpeedY = canMove ? (isRunning ? runningSpeed : walkingSpeed) * Input.GetAxis("Horizontal") : 0;
         float movementDirectionY = moveDirection.y;
         moveDirection = (forward * curSpeedX) + (right * curSpeedY);
 
         if (Input.GetButton("Jump") && canMove && characterController.isGrounded)
         {
             moveDirection.y = jumpSpeed;
         }
         else
         {
             moveDirection.y = movementDirectionY;
         }
 
         // applies gravity
         if (!characterController.isGrounded)
         {
             moveDirection.y -= gravity * Time.deltaTime;
         }
 
         // move the controller
         characterController.Move(moveDirection * Time.deltaTime);
 
         // player and camera rotation
         if (canMove)
         {
             rotationX += -Input.GetAxis("Mouse Y") * lookSpeed;
             rotationX = Mathf.Clamp(rotationX, -lookXLimit, lookXLimit);
             playerCamera.transform.localRotation = Quaternion.Euler(rotationX, 0, 0);
             transform.rotation *= Quaternion.Euler(0, Input.GetAxis("Mouse X") * lookSpeed, 0);
         }
     }
 }

I don't have experience using Unity and this is literally my first project. Any help is very much welcome! thank you.