Main Menu script not working...

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

public class MainMenu : MonoBehaviour
{
    public void PlayGame ()
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
    }

    public void QuitGame ()
    {
        Application.Quit();
    }
}

Assets\Scripts\MainMenu.cs(8,17): error CS0111: Type ‘MainMenu’ already defines a member called ‘PlayGame’ with the same parameter types

Assets\Scripts\MainMenu.cs(6,14): error CS0101: The namespace ‘’ already contains a definition for ‘MainMenu’

I have no clue what global namespace is

Sounds like you potentially have duplicate script files in your project (two MainMenu scripts/classes).