I'm having the same issue. All my builds were "downloadable" through TestFlight. But for a certain build—with no significant differences from the previous ones (just style and wording, but no new dependency or capability) and from this build until now, I have the error: 'Could not install app-name. The requested app is not available or doesn't exist.' I've contacted support by phone and then by email last week, but I haven't received an answer so far. Can anyone relate or have any idea how to resolve this?