AddListener issue C#

I am not sure why it keeps telling me the onClick is not referred to. I am trying to understand.

http://pastebin.com/4ybyiDxf

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 
 public class PuzzleGameController : MonoBehaviour {
 
     [SerializeField]
     private Sprite BgImage;
 
     public Sprite[] puzzles;
     public List<Sprite> gamePuzzles = new List<Sprite>();
 
     public List<Button> btns = new List<Button>();
 
     private bool firstGuess, secondGuess;
 
     private int countGuesses;
     private int countCorrectGuesses;
     private int gameGuesses;
     private int firstGuessIndex;
     private int secondGuessIndex;
 
     private string firstGuessPuzzle, secondGuessPuzzle;
 
     void Awake()
     {
         puzzles = Resources.LoadAll<Sprite>("Sprite");
     }
 
     // Use this for initialization
     void Start () {
 
         GetButtons();
         AddListeners();
         AddGamePuzzles();
 
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void GetButtons()
     {
        
         GameObject[] objects = GameObject.FindGameObjectsWithTag("LetterButton");
 
         for (int i = 0; i < objects.Length; i++)
         {
             //add buttons to the grid
             btns.Add(objects[i].GetComponent<Button>());
             //change image on each button
             objects[i].GetComponent<Image>().sprite = BgImage;
 
         }
         
     }
 
     void AddListeners()
     {
 
         foreach (Button btn in btns)
         {
             btn.onClick.AddListener(() => PickAPuzzle());
         }
     }
 
     void AddGamePuzzles()
     {
         int looper = btns.Count;
         int index = 0;
 
         //loop by how many buttons you have divide by 2 to make new row
 
         for(int i = 0; i < looper; i++)
         {
             if(index == looper / 2)
             {
                 index = 0;
             }
 
             gamePuzzles.Add(puzzles[index]);
             index++;
         }
     }
 
 
     public void PickAPuzzle()
     {
 
         string name = UnityEngine.EventSystems.EventSystem.current.currentSelectedGameObject.name;
        Debug.Log("Button Clicked " + name);
         Debug.Log("first " + firstGuess);
         Debug.Log("second " + secondGuess);
         if (!firstGuess)
         {
             firstGuess = true;
             Debug.Log(firstGuess);
             //find index of button clicked on
             firstGuessIndex = int.Parse(name);
             Debug.Log("Button Clicked " + name);
             Debug.Log(firstGuessIndex);
             btns[firstGuessIndex].GetComponent<Image>().sprite = gamePuzzles[firstGuessIndex];
 
 
         }
         else if(!secondGuess){
             secondGuess = true;
             Debug.Log(secondGuess);
             //find index of button clicked on
             secondGuessIndex = int.Parse(name);
             Debug.Log("Button Clicked " + name);
             Debug.Log(secondGuessIndex);
             btns[secondGuessIndex].GetComponent<Image>().sprite = gamePuzzles[secondGuessIndex];
 
         }
     }
 }