If you opt to pay with a bank card your payment is done via Maestro, which is an international payment method.
In some countries and with certain types of card there is a verification code on the card, but that is not the case in Belgium or most European countries.
You can therefore simply leave the field for the verification code blank.
Note
If you use a credit card such as Visa or MasterCard, there is often a code on the back of the card as well, but credit cards are not accepted on Doccle.