Facebook Messengerbot python ( The Callback URL or Verify Token couldn't be validated. Please verify the provided information or try again later.)

Am trying to implement facebook messenger chatbot in python. Am created One API in python below

@api_view(['GET'])
def verify(request):
    form_data = request.query_params
    mode = form_data.get('hub.mode')
    token = form_data.get('hub.verify_token')
    challenge = form_data.get('hub.challenge')
    if mode and token:
        if mode == 'subscribe' and token == "mytestingtoken":
            print("WEBHOOK_VERIFIED")
            return JsonResponse({"code":200,'message':challenge})
        else:
            return JsonResponse({"code":403})
    return JsonResponse({"code":200,'message':'test'})

am mapping the API URL into ngrok URL(https://55d71a8248be.ngrok.io)

then am created a Facebook App and configured a webhook.
here callback URL and Verify Token also setup.
but finally I got Error Message

The Callback URL or Verify Token couldn't be validated. Please verify the provided information or try again later.

Am referred facebook document https://developers.facebook.com/docs/messenger-platform/getting-started/webhook-setup