SpriteRenderer doesn't show the sprite

Hi I'm trying to change the image of the sprite when I press some letter, this is my code

 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class juegodiferencias : MonoBehaviour {
     public GameObject Canvas;
     public GameObject Panel;
     public GameObject ImagenCambiante;
     public GameObject createImage;
     public string random; 
     public char ascii;
     public string rletra;
 
     // Use this for initialization
     void Start () {
         
 
         createImage = Instantiate(ImagenCambiante, new Vector3 (0, 0, -1), new Quaternion (0,0,0,1)) as GameObject;
         createImage.transform.SetParent (Panel.transform, false);
         Panel.transform.SetParent (Canvas.transform, false);
 
         random = Random.Range (1, 6).ToString();
         ascii = (char)Random.Range (97, 99);
         rletra = ascii.ToString ();
 
 
     }
 
     // Update is called once per frame
     void Update () {
 
         if (Input.GetKeyDown ("space")) {
             createImage.GetComponent<SpriteRenderer>().sprite = Resources.Load<Sprite> ("Sprites/"+random+"/"+random);
         
 
         }
         if (Input.GetKeyDown ("w")) {
             createImage.GetComponent<SpriteRenderer>().sprite = Resources.Load<Sprite> ("Sprites/"+random+"/"+random+rletra);
 
         }
 
     }
 
 }

When I press the letters nothing appears but in the preview board it could see that the sprite was assigned

Thank you for your help.