Authentication Required
What it means
The transaction requires 3D Secure (3DS/SCA) authentication that was not completed.
Why it happens
The issuer requires the cardholder to authenticate (for example via 3D Secure), but authentication was not performed or did not succeed.
If you’re the cardholder
Complete the authentication step (for example the bank's 3D Secure prompt) and retry.
If you’re the merchant
Trigger the 3D Secure / SCA authentication flow and resubmit once the customer authenticates.