Some credit and debit cards have an additional security feature, such as Visa’s 3-D Secure or Mastercard’s SecureCode.
In these cases, additional verification is required by the cards issuing bank to ensure that the card is being used by the cardholder, to reduce the risk of fraud. How the bank performs this security measure is up to them and is not controlled by MySudo.
In some cases, your bank may send you an SMS or call you with a code. In other cases, no interaction will be required if you are using MySudo from a location the bank considers low-risk for your account.
Using a VPN can sometimes trigger these security features, you may need to try a different egress location or disconnecting the VPN while trying to add your funding source.