CS0618 warning

I wanted to make a button called "Play" that would change the scene from "Menu" to "Game Modes" and only get this one error. I am asking for help as soon as possible.

This is my script:
using UnityEngine;
using System.Collections;

public class Menu : MonoBehaviour
{

void OnGUI()
{
if (GUI.Button(new Rect(10, 70, 50, 30), "Button Play"))
{
Application.LoadLevel("tryby gry");
}
}
}