3D Secure (Three-Domain Secure) is a secure user authorization protocol for CNP transactions (without a card present), which is used as an additional level of security for making payments for goods and services online.
This error means that 3DSecure service - confirmation of payment by one-time password code - is not available for the card you are trying to pay with
Failure code: 321 (3DSecure authentication failed.)
And in this case a refusal is received from the bank.
If the error occurs, we recommend to use another card or another payment method presented on the website, for example PayPal, or contact the bank's support to clarify the possibility of enabling 3DSecure.