Same issue for my team (Team ID 4ZUS8ZLZD8), starting Oct 20, 2026. The TestFlight builds of all my apps switched to "Build Removed" at about the same time.
New builds (Xcode 27.1, iOS 27.1 SDK) process fine ("Valid", "ready to test" email), but every install fails with "The requested app is not available or doesn't exist." The same archive installs and runs fine via Xcode / devicectl (development signing). Device: iPhone 15, iOS 27.0.1.
Agreements took effect Oct 13 / Oct 16 and were accepted Oct 18; membership active until January 2027. Already tried: re-adding myself to the internal group, reinstalling TestFlight, signing out/in, restarting.
Feedback: FB25271158. Developer Support case: 103003744026.