I'm having the same problem with all my apps, not just one. Internal TestFlight builds fail to install on both my iPhone and iPad. On the iPhone, the error is "The requested app is not available or doesn't exist"; on the iPad, it says "Try again later", and tapping "View" does nothing. I've checked that the builds are assigned to the internal testing group, show as Testing and haven't expired. My tester account and developer agreements are active. Restarting the iPhone and switching from Wi-Fi to cellular data made no difference. I contacted Apple Developer Support on November 3 and received an acknowledgement, but no further response so far. Has anyone had this resolved by Apple?