You can try
if  let sprites = pokemonDictionary["sprites"] as? [String:Any] {
      print(sprites)
   if  let backImgUrl = sprites["back_default"] as? String {
        print(backImgUrl)
    }
}
Also you should run function named CallUrl in a thread other than the main , as Data(contentsOf:) runs synchronously and blocks the main thread
1
solved Nested Dictionary in my swift application