Transaction Error: Transaction was blocked by the Payment Server because it did not pass all risk checks

What does 'Transaction was blocked by the Payment Server because it did not pass all risk checks' mean?

This means that the customer's card is not registered for 3DS authentication which is required before a transaction can be completed.

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.