When you are using Adyen’s Apple Pay certificate, and receive the 422 Unauthorized to process transactions on behalf of merchantId error when attempting an Apple Pay transaction, this means that there is an issue with requesting your merchant session token from the /applePay/sessions endpoint.
This error can be a result of a problem with your integration, or with the configuration of Apple Pay on your merchant account. You can find more information on how to resolve this error here based on your integration type.