Transaction Error: The cardholder was not authorised. This is used in 3-D Secure Authentication

What does 'The cardholder was not authorised. This is used in 3-D Secure Authentication' mean?

This means that 3D secure authentication was attempted on the card, but the customer's card isn't enrolled.

3DS also known as 3DSecure or 3-D Secure Authentication is an extra level of security to ensure that every transaction done with your card is authorized by the cardholder. This is put in place to reduce fraudulent activities for Card Not Present (CNP) transactions.

What can I do about it?

Kindly advise your customer to contact their bank to register for Mastercard Secure code for Mastercard or Verified by Visa for Visa cards.