Before loading ads, have your app initialize the SDK X

GreedyGameAds.Instance.Init();

using UnityEngine;
using GreedyGame;

 public class GreedyGameAdsDemoScript : MonoBehaviour
 {
    private void Start()
    {
       Initialize();       
    }

    public void Initialize()
    {    
       GreedyGameAds.Instance.Init();
    }
 }

 GreedyGameAds.Instance.Init(callback);

 public void Initialize()
 {    
   GreedyGameAds.Instance.Init(callback);
 }

 private void callback(bool isInitialized, string message)
 {
    if (isInitialized)
    {
        //Debug.Log( "Initialization" + message );  
    }
    else
    {
        //Debug.Log( "Initialization" + message );     
    }
 }