Update (Oct 28): the problem now also affects internal testing. Internal testers on my team can no longer install any build of any of my apps from TestFlight. On an iPhone 17 Pro Max with iOS 27.0.1, tapping Install or Update shows "Could not install. The requested app is not available or doesn't exist." This happens for two different apps on the same team. In App Store Connect the builds show VALID, not expired, internal state IN_BETA_TESTING, and the tester is in the internal group with access to all builds. This looks like an account-level problem.