Anybody out there an expert in C#?

I'm having an issue right now while trying to follow a course for Unity. The course goes through the code that is currently outdated and I have a more recent version of it. The issue is as followed: I need to make a playerpref for unlocking new levels in the game and I am being instructed to use :

if (level <= Application.levelCount - 1)
{
return isLevelUnlocked;}

since this is obsolete, I'm not sure what to use in place of it... I saw some people suggest to use the scenemanagment, but I can't get it to work... here is the full coded:

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

public class PlayerPrefsManager : MonoBehaviour
{

const string MASTER_VOLUME_KEY = "master_volume";
const string DIFFICULTY_KEY = "difficulty";
const string LEVEL_KEY = "level_unlocked_";


public static void SetMasterVolume(float volume)
{
    if (volume > 0f && volume < 1f)

    {
        PlayerPrefs.SetFloat(MASTER_VOLUME_KEY, volume);
    }
    else
    {
        Debug.LogError(" Master volume out of range");
    }

}


public static float GetMasterVolume()
{
    return PlayerPrefs.GetFloat(MASTER_VOLUME_KEY);
}

public static void UnlockLevel(int level)
{
    if (level <= Application.levelCount - 1)
    {
        PlayerPrefs.SetInt(LEVEL_KEY + level.ToString(), 1);


    }
    else
    {
        Debug.LogError("trying to unlock level not in build order");
    }

}
public static bool IsLevelUnlocked(int level)
{

    int levelValue = PlayerPrefs.GetInt(LEVEL_KEY + LEVEL_KEY.ToString());
    bool isLevelUnlocked = (levelValue == 1);

    {
        if (level <= Application.levelCount - 1)
        {
            return isLevelUnlocked;
        }
        else
        {
            Debug.LogError("trying to query level not in build order");
            return false;
        }



    }
}
public static void SetDifficulty(float difficulty)
{
    if (difficulty >= 0f && difficulty <= 1f)
    {
        PlayerPrefs.SetFloat(DIFFICULTY_KEY, difficulty);
    }
    else
    {
        Debug.LogError("difficulty out of range");
    }
}

public static float GetDiffculty()
{
    return PlayerPrefs.GetFloat(DIFFICULTY_KEY);
}

}

with the course, I test it using PlayerPrefsManager.UnlockLevel(2); , but it wont work because the damn thing is outdated and I haven't the foggiest on how to compensate for it... any clues?