I integrated Apple Pay on the web following the documentation, and verified that the payment works successfully in the sandbox environment using Safari on iOS and Mac. Later, I launched the Apple Pay payment code on a third-party web page that supports Apple Pay, and scanned the code with the camera of an eligible iOS device. The camera then displayed the Apple Pay button; after tapping it, a brief pop-up message saying ‘Service Unavailable’ appeared, and then it closed automatically. How can I troubleshoot and resolve this issue?