Assets/Scripts/MouseMovement.cs

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

public class MouseMovement : MonoBehaviour
{

  public float mouseSensitivity = 100f;

  float xRotation = 0f;
  float YRotation = 0f;

  void Start()
  {
    //Locking the cursor to the middle of the screen and making it invisible
    Cursor.lockState = CursorLockMode.Locked;
  }

  void Update()
  {
    if (!InventorySystem.Instance.isOpen && !CraftingSystem.Instance.isOpen)
    {

      float mouseX = Input.GetAxis("Mouse X") * mouseSensitivity * Time.deltaTime;
      float mouseY = Input.GetAxis("Mouse Y") * mouseSensitivity * Time.deltaTime;
      // Debug.Log("Mouse X: " + mouseX);
      //control rotation around x axis (Look up and down)
      xRotation -= mouseY;

      //we clamp the rotation so we cant Over-rotate (like in real life)
      xRotation = Mathf.Clamp(xRotation, -90f, 90f);

      //control rotation around y axis (Look up and down)
      YRotation += mouseX;

      //applying both rotations
      transform.localRotation = Quaternion.Euler(xRotation, YRotation, 0f);
    }
  }
}