Same problem happening here. I was able to add a sandbox user about a year ago, tried today and I got that error.

Got into contact with Apple support. We can't add a user to sandbox testers if that user has a real Apple ID. So add an email address that hasn't been attached to an Apple ID.

Hi there, this is interesting because I am experiencing the same issue with my node.js server. Here is my code: const body = { "receipt-data": receipt, password: APPLE_IAP_SECRET, "exclude-old-transactions": true, }; const ret = await fetch(url, { method: "POST", headers: { "Content-Type": "application/json" }, body: data }); const parsed = await ret.json(); return parsed; Where url is either https://buy.itunes.apple.com/verifyReceipt or https://sandbox.itunes.apple.com/verifyReceipt . Pretty sure the object body is correct json, as this is being coded in javascript. @tomtom1 Not too sure what you mean by "i should pass a String object to the POST request".. ?

@tomtom1 if I JSON.stringify(body) it goes through perfectly! Thanks for the help :)