In my case, that was an email that was from a domain Apple did not like. When I switched to a more "common" account (something gmail related) it accepted my users.

I'm encountering the exact same issue. No error in the UI, error 500 with a json object : { "validationResults": [ { "field": null, "localizedField": null, "validationRuleName": null, "messageKey": null, "error": null, "propertyName": null, "errorShortMessage": null, "errorFlag": null, "errorString": "An unknown error has occurred.", "errorNumber": -21607 } ], "status": null, "code": null } https://appstoreconnect.apple.com/sandbox/v2/account/create