Completely uninstall iOS App and all associated data

Please try the following:

1. Go to Settings --> General --> (iPhone/iPad/iPod/ect.)Storage --> Select app and "Delete App"

2. Go to Settings --> Profiles & Device Management (Could also be called "Profiles", if you don't use this feature, it will not be there) and delete any app data.

3. Go to Settings --> [Your Name] (At the very top) --> iCloud --> Manage Storage --> Select the app and "Delete Data"

4. Delete and re-install the "Test Flight" app

5. Completely reset the device (Press and hold Home and Sleep until the Apple logo appears)

5a. If this device is only for testing, you can go to Settings --> General --> Reset --> Reset All Settings

6. Reinstall the app

Let me know if it works!