Coroutine not starting

Hello. Why coroutine not started?

I use Firebase Realtime database to handle some data in my Unity mobile application It's work fine when i get url and press 'S'.

But I want start coroutine from code. Can anyone see what I am doing wrong? Thanks.

 using Firebase;
 using Firebase.Database;
 using Firebase.Unity.Editor;
 using UnityEngine;
 using UnityEngine.UI;
 using SimpleJSON;
 using UnityEngine.Networking;
 using System.Collections;
 using Firebase.Storage;
 using System.Threading.Tasks;
 using System;
 
 public class Connector : MonoBehaviour
 {
     [SerializeField] string _db;
     [SerializeField] int _cuurentIndex; //current page index
     [SerializeField] RawImage _rawTexture;
     FirebaseStorage _storage;
     [SerializeField] string _url;
 
 
     void Start()
     {
         FirebaseApp.DefaultInstance.SetEditorDatabaseUrl(_db);
         _storage = FirebaseStorage.DefaultInstance;
 
         FirebaseDatabase.DefaultInstance
         .GetReference(_cuurentIndex.ToString())
         .GetValueAsync().ContinueWith(task =>
         {
             if (task.IsFaulted)
             {
                 print(task.Exception.Message);
             }
             else if (task.IsCompleted)
             {
                 DataSnapshot snapshot = task.Result;
                 print(snapshot.GetRawJsonValue());
                 ParseJson(snapshot.GetRawJsonValue());
             }
         });
     }
 
 
     // THIS WORKING WHEN I GET URL AND PRESS 'S'
     private void Update()
     {
         if (Input.GetKeyDown(KeyCode.S))
         {
             StartCoroutine(GetTexture(_url));
         }
     }
     
 
     private void ParseJson(string jsonString)
     {
         var N = JSON.Parse(jsonString);
         print(N["url"].Value);
         StorageReference gs_reference =
         _storage.GetReferenceFromUrl(N["url"].Value);
 
         gs_reference.GetDownloadUrlAsync().ContinueWith((Task<Uri> task) =>
         {
             if (!task.IsFaulted && !task.IsCanceled)
             {
                 Debug.Log("Download URL: " + task.Result.OriginalString);
                 _url = task.Result.OriginalString; // for test
                 StartGetTexture(task.Result.OriginalString);
             }
         });
     }
 
 
     private void StartGetTexture(string url)
     {
         StartCoroutine(GetTexture(url));
     }
 
 
     IEnumerator GetTexture(string textureUrl)
     {
         UnityWebRequest www = UnityWebRequestTexture.GetTexture(textureUrl);
         yield return www.SendWebRequest();
 
         if (www.isNetworkError || www.isHttpError)
             Debug.Log(www.error);
         else
             _rawTexture.texture = ((DownloadHandlerTexture)www.downloadHandler).texture;
     }
 }