Sometimes you might add your Zestful card to the billing page of a website or online purchase and it immediately comes back as incorrect or failed. Generally this is because the information you entered is not matching the billing information in your Zestful account.
When you create a Zestful account, it is important to understand that the billing address entered in your profile is used as the billing address for purchases. So, if you are ever asked for a zip code (or the entire address) when making a purchase, your Zestful billing address is what is referenced. Make sure the address matches your purchase, or change it in your profile if necessary.
Card number, CVC, or expiration date mismatch
Be sure the CVC number matches your Zestful card CVC located on the back of the card (Or on the My Cards screen if it is your virtual card).
Physical card not activated
Sounds obvious, but be sure to activate your physical card prior to use! Once you receive the physical card in the mail, you need to log into Zestful and click the "Activate" button on the top right.