Yes, the answer is I AM AN IDIOT... pure and simple.

I was setting my user account in itunes login to these test accounts.. which invalidates them as test accounts because they try to legitimately log into itunes...

You have to have your ITUNES/APPstore login EMPTY, do not try to log in explicitly in the settings... LOG OUT of ITUNES/APP STORE so that no account is in the apple ID in settings and then just run your app & login the test account via your app.

I had properly done this, but then without reconsulting the documentation I went to change users, and saw that I had a test user in the UI of the settings (I guess it gets set by the software) so I attempted to just set it there, and was invalidating the accounts causing this problem.

This is purely my mistake, but Apple could do a better job on the error this thing throws, and have the app store/itunes perhaps display something other than the test id, so if its been a week since or two since you set your test id and go to change it you don't just foolishly think you do it that way...

Anyway... you will likely need to delete all your sandbox testers if you did what I did as they will be invalid and then create new ones... and then remove the appstore/itunes login on your test device and try to run it....

I admit this is documented, I did follow it the first time I used a test user, but forgot the next time I wanted to test.. which lead to this... fully my fault.. but I suspect I am not, nor will I be the only one to do this... which is why it's so odd I couldn't find anything else documented about this error anywhere.

Documentation for reference:

• current: https://developer.apple.com/documentation/storekit/in-app_purchase/testing_in-app_purchase_transactions
• archive: https://web.archive.org/web/20171108103850/https://developer.apple.com/library/content/documentation/LanguagesUtilities/Conceptual/iTunesConnectInAppPurchase_Guide/Chapters/TestingInAppPurchases.html
  (Make sure to expand the : To perform testing for In-App Purchase products section)

In my case, just log out your real Apple ID, don't login sandbox tester Apple ID, then run your App, buy product, and system will showing a alert view to let you choose "Use Existing Apple ID" or "Create New Apple ID", of course choose first one, then login your sandbox tester Apple ID, and it's done.

Answer 2020

If your iOS is after 13, now go to Settings -> iTunes & App Storesat the bottom you will see a section called:

SANDBOX ACCOUNT

this is the place where you can log in your tester accounts

below screenshot shows where it is: