Apple In App Payments: Sandbox user issue

Possible short answer: "Try creating a new sandbox user in Apple's developer portal and use it for testing in-app payments."

Related articles:

Troubleshooting Apple In-App Payment Sandbox User Issues
Apple’s In-App Purchase (IAP) Sandbox allows developers to test the functionality of their apps before releasing them to the public. This is a vital feature that helps developers ensure their apps are secure, fast, and user-friendly. However, it’s not uncommon for users to experience issues while using the IAP Sandbox. Here, we’ll take a look at some of the most common user issues and how to resolve them.

Common Problems with Apple In App Payment Sandbox User Testing
Apple provides a sandbox environment for developers to test their in-app payment processes before submitting their apps to the App Store. While this is a useful tool, there are some common problems that developers may encounter during this testing period.

How to Fix In App Payment Sandbox User Issues on Apple Devices
If you are having issues with in-app payment sandbox user on your Apple device, you are not alone. Many users have reported encountering issues when trying to make payments on applications where sandbox user is involved.