If you get this error when placing an order it's possible that the credit card you have entered has asked you to specifically enter your postcode for the order.
This entry field may not show when you initially try to place your order and will appear after you have entered your details, maybe further in your order.
This field will be separate from your order address details, where you have probably already entered your postcode already. It will appear next to your credit card enter and you may miss it, if you are further down the order.
We have illustrated this for you: -