As of September 14, 2019, the Revised Payment Service Directive, also known as PSD2, will be implemented in all countries in the European Economic Area (EEA). PSD2 requires most online transactions within the EEA to have customer authentication. To be compliant, Shopify will process credit card transactions in the EEA by using 3D Secure, which is a payment authentication method.
If your app or sales channel implements either the Checkout API or the Storefront API, consider updating it to the 2019-10 release candidate API version of the Checkout API or Storefront API so that it can fully support payment authentication using 3D Secure.
To learn more about changes that you can make to support 3D Secure, refer to Authenticating payments with 3D Secure.