This article explains why a test credit card fails in a Sandbox environment with PayPal APIs.
When using a test Visa credit card
4111 1111 1111 1111 from PayPal, sometimes it fails due to PayPal fraud policies with the following error::
Error happened when processing the request. Please try again later.
This error is displayed when PayPal flags a specific test credit card number for fraud. This happens because it is a test credit card number used by everyone around the world testing with PayPal APIs.
Use another test credit card. To generate mock credit cards you can use for testing: