Load level once

The below script is my main menu button.

How it works: That button loads a new level each time a level is passed. E.g. I pass level 2 and get to level 3, i quit game and click 'Play Game', then level 3 would load.

My problem: When I built my game and tested, it would not load any level, as it only loads a level each time it is passed. How would I add something to tell it to load level 1 when game is just downloaded, then to continue doing my following script.

Any advice or script example would be appreciated, many thanks

     public void PlayGame()
     {
         loadingImage.SetActive(true);
         if (PlayerPrefs.GetString("Level").Equals(null))
         {
             SceneManager.LoadScene(levelSelect);
         }
         else
         {
             levelSelect = PlayerPrefs.GetString("nLevel");
             SceneManager.LoadScene(levelSelect);
         }
         
     }