Same problem... We do new release build and the release build is process in Testflight. After processing internal testers in testflight are informed about a new release build but when they try to install same error message as you have: "The requested app is not available or doesn't exist" We have tried several things. Did delete the internal tester group and did invite test users again. No success... Did open support request at Apple. Please let me know how could solve the issue.