Apple Pay Sandbox Testing - ACCOUNT_UNAVAILABLE

Hi.

I am currently attempting to implement Apple Pay for Web, which will integrate with a third party payment gateway. The testing requires that you use a Sandbox Apple ID alongside a sandbox wallet and test card in order to verify the integration, and here's where I'm having a problem.

My colleague has created a Sandbox Tester account for me, for which I received the verification email and did the necessary to verify it. I do not own an iPhone, and it does not appear that the Simulator which ships with Xcode allows you to add cards to the wallet (unless I'm missing that due to the issues I will go on to describe shortly) so I tried to log this sandbox Apple ID into my Macbook running the latest release of Big Sur.

If I try to log into the icloud.com or appleid.apple.com websites, the log in is fine. 2FA is enabled and I either get the text message or the on-screen six digit code. No problems here.

If I try to log into macOS, my username and password are accepted (indeed, it was here where I first set up the 2FA) but then a message box appears saying ACCOUNT_UNAVAILABLE. I get the same sequence of events and same error if I try to log in through System Preferences after reaching the desktop, unable to log in during account setup.

If I try to log into the App Store, I get an error saying the account can't be used with the iTunes store or similar - I don't have the exact wording.

In the simulator (if my previous attempt is anything to go by - I asked my colleague to set up a new sandbox account from scratch for the one that's led to this email) it appears to accept my login but won't allow the setup of Media & Purchases, and Wallet just displays a message about downloading suitable apps that goes nowhere when clicked - no options to add a card.

In summary:

• Created a new sandbox account through ADC and verified the email address
• Can sign in to icloud.com
• Can sign in to appleid.apple.com
• CANNOT sign in to macOS during first login (ACCOUNT_UNAVAILABLE)
• CANNOT sign in to macOS through system preferences (ACCOUNT_UNAVAILABLE)
• CANNOT sign in through App Store (message regarding iTunes)
• CANNOT set up Wallet / Apple Pay (since I can't log in)
• Simulator doesn't allow addition of cards even though it does appear to accept login

There appears to have been a few people in the past few days reporting seeing a similar message although I'm unsure if they're all trying to do things with sandbox accounts as well. If anyone can help shed any light on this so that I can complete my integration and get this payment gateway signed off, I would very much appreciate it.