By contrast, I can sign in to the Sandbox Apple Account without issue on a physical device. The problem occurs only via Simulator. That is the expected behavior. To test in the sandbox environment, you need a physical device. For more information, see Testing In-App Purchases with sandbox. StoreKit Testing in Xcode is a local test environment for testing in-app purchases without requiring a connection to App Store servers. You can use it to test In-App Purchases in the Simulator without a Sandbox account.
Topic:
App & System Services
SubTopic:
StoreKit
Tags: