Level Selection and Strange Error

I followed this great tutorial video on level selection that doesn't let you move forward unless you have at least 1 out of 3 stars and it also keeps track of these stars. Sadly the code simply isn't working.

The error I keep getting is this

FormatException: Input string was not in a correct format.

System.Number.StringToNumber (System.String str, System.Globalization.NumberStyles options, System.Number+NumberBuffer& number, System.Globalization.NumberFormatInfo info, System.Boolean parseDecimal) (at :0)

System.Number.ParseInt32 (System.String s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) (at :0)

System.Int32.Parse (System.String s) (at :0)

LevelSelection.UpdateLevelStatus () (at Assets/Scripts/LevelSelection.cs:23)

LevelSelection.Update () (at Assets/Scripts/LevelSelection.cs:18)

Here's the code, thank you to anyone who can help.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 public class LevelSelection : MonoBehaviour
 {
     [SerializeField] private bool unlocked;
     public Image unlockImage;
     public GameObject[] stars;
 
     public Sprite starSprite;
 
     private void Update()
     {
         UpdateLevelImage();
         UpdateLevelStatus();
     }
 
     private void UpdateLevelStatus()
     {
         int previousLevelNum = int.Parse(gameObject.name) - 1;
         if (PlayerPrefs.GetInt("lv" + previousLevelNum.ToString()) > 0)
         {
             unlocked = true;
         }
     }
 
     private void UpdateLevelImage()
     {
         if(!unlocked)
         {
             unlockImage.gameObject.SetActive(true);
             for(int i = 0; i < stars.Length; i++)
             {
                 stars[i].gameObject.SetActive(false);
             }
         }
         else
         {
             unlockImage.gameObject.SetActive(false);
             for(int i = 0; i < stars.Length; i++)
             {
                 stars[i].gameObject.SetActive(true);
             }
         }
     }
 
     public void PressSelection(string _LevelName)
     {
         if(unlocked)
         {
             SceneManager.LoadScene(_LevelName);
         }
     }
 }
 
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class SingleLevel : MonoBehaviour
 {
     private int currentStarsNum = 0;
     public int levelIndex;
 
     public void BackButton()
     {
         SceneManager.LoadScene("LevelSelection");
     }
 
     public void PressStarsButton(int _starsNum)
     {
         currentStarsNum = _starsNum;
 
         if (currentStarsNum > PlayerPrefs.GetInt("Lv" + levelIndex))
         {
             PlayerPrefs.SetInt("Lv" + levelIndex, _starsNum);
         }
 
         //BackButton();
         //MARKER Each level has saved their own stars number
         //CORE PLayerPrefs.getInt("KEY", "VALUE"); We can use the KEY to find Our VALUE
         Debug.Log(PlayerPrefs.GetInt("Lv" + levelIndex, _starsNum));
 
         BackButton();
     }
 
 }