This seems like an age-old error that I've never found the answer to. I created a simple app with a button that changes a text field for a demonstration of the TestFlight app and have never gotten the internal test to work. Created another simple app. Still can't download the app in TF.
I've tried just about everything, but when I choose install in the TF app I get the error. I'm the only person listed in App Store Connect as a developer, so I chose myself as the only internal tester in the group.
I've tried various builds also. In TF under app details it shows that my test apps will work on my phone. Everything seems in order when I push my archived apps from XCode to the App store. Showing up as Ready to test in the TestFlight section of the App store.
I have no clue what to try next, other than moving on to external testers - which looks even more complicated to configure.