Dennis is an audiophile and he's having issues playing music from smartphone using Google Chromecast. It wants him to upload all his music to the cloud first. Leo says that his casting app may be corrupted. He should try another one, like AllCast. From the chatroom - Google requires you to upload your music to the cloud before casting with Google Play. That's what it's designed for: to play from the phone at home using the Google Home App.
Sam Abuelsamid also says that LocalCast is the app Dennis will want to use to cast from his smartphone. It will cast any media that's local.