The requested app is not available or doesn't exist.

You mean that you have invited yourself, already? If that's case, the only reason I can think of is that you may be using a wrong email address. If I remember correctly, you cannot use your Apple ID email address to test-flight your apps. I can be wrong. I haven't used TestFlight for 4 or 5 years.

I have the same problem. About 70 people were testing the application via TestFlight. A mandatory update was required. The application works in the simulator and on a real device. On TestFlight, I can't share it with the external group, and I encounter errors during download. I updated Xcode, renewed the certificates, reviewed the agreements, and reinstalled everything from scratch. The problem still persists.

The previous working versions expired during the initial installation.

It's stuck at the Ready to Submit stage. I wrote to support and am waiting for a response.

@savci2 Thanks for your post, this is the second old thread I see you posting the issue you are having with TestFlight. I would recommend you if you are having a problem with TestFlight to create a new thread and post all the logs and screenshots so engineers here can look if this is a new issue or an existing issue and help you.

Make sure you’ve added the external testers’ emails to the Internal Testing and External Testing lists in the App Store Connect dashboard. External testers must accept the invitation to download the app.

Ensure that all agreements related to distributing your app on TestFlight are active.

Check the logs in Xcode’s Devices and Simulators section. There might be error messages related to the download issue and provide those logs on your post.

Hope this helps.

Albert Pascual
  Worldwide Developer Relations.