Further update! Apparently checkout with an apple id that's part of an organisation's internal testing group (NOT sandbox test accounts) will work on testFlight build. Builds approved for external testing may also work with your current apple ID Test accounts are bricked though.

Been stuck on this since morning, broken across old and new builds. https://developer.apple.com/system-status/ shows everything as green but not been able to get IAP working.

Same here for xcode 11.3.1 ... same build uploaded fine on on 11th of September 2020