Yes, it's sandboxed apps for the app store.

I found a solution: https://developer.apple.com/forums/thread/128166

I am facing exactly the same problem with a Qt macOS app. The app can be released to the store, works fine everywhere, but TestFlight is not available "Not Available for Testing". The binary data is as follows: Interestingly the "Build SDK" and "Build Platform" fields are empty. Has anyone an idea/suggestion how to get TestFlight to work? Are there specific prerequisits necessary (for instance a minimum macOS version, specific signing, etc.)?

Finally I got testflight working by adding the "com.apple.application-identifier" and "com.apple.developer.team-identifier" tags to the app.entitlements file. Now, TestFlight is enabled but the app crashes with a "Code signature invalid" crash.

After spending more than a day with this I finally found the reason: https://github.com/microsoft/vscode/issues/113720 In short: the crash is expected but the app will work when installed via TestFlight.

It turned out that xpc_connection_create_mach_service does not work with QuickTime.

Did you ever come up with a solution to this? I am facing exactly the same problem - removing of languages works but as soon as I press the "Save" button, "You have one or more errors on this page." is shown.

Thanks, I got it working (also in the sandbox). It was a fault on my side.