Maestro cards operate differently from regular Visa and MasterCard credit cards. Indeed, they have 17 digits instead of 16. Moreover, they don't have any CVV number (security code at the back of the card). However, this type of card can be used with PayPlug.
About the CVV, some banks allow you to use the code "000" or "999". If those numbers don't work, we recommend that you contact your bank and ask them some details on how to use your card for online purchases.
You also need to fill in the 17 digits of your card.
Unfortunately the Belgian Maestro Bancontact cards are not allowed because they are part of a specific network.
You can find more information about these cards on the official Maestro FAQ.