The compiler throws error CS1520 error: the method must have a return type please help

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using TMPro;
using UnityEngine.Events;
using UnityEngine.EventSystems;
using UnityEngine.SceneManagement;

public class Ahivments : MonoBehaviour
{
    int total_Coin_Vanya;

    [SerializeField] Button firstAch;
    [SerializeField] bool isFirst;


    // Start is called before the first frame update
    void Start()
    {
    total_Coin_Vanya = PlayerPrefs.GetInt("total_Coin_Vanya");
    if(total_Coin_Vanya2 >= 10 && !isFirst)
    {
     firstAch.interactable = true;
    }
    else
    {
     firstAch.interactable = false;
    }
    }

    public GetFirst()
    {
    int Coin_Vanya = PlayerPrefs.GetInt("Coin_Vanya");
    Coin_Vanya =+ 10;
    PlayerPrefs.SetInt("Coin_Vanya", Coin_Vanya);
    isFirst = trye;
    PlayerPrefs.SetInt("isFirst", isFirst ? 1 : 0);
    }

    public void ToMeinMenu()
    {
    SceneManager.LoadScene(0);
    }
}