How to add error message when zip-code is wrong? VirtueMart 1.1.x

I don’t do any coding so don’t know how to add an error message . We get an error message after customer tries to do the payment, since payment is not accepted by the “bank” if customer has added too long/short postal code (see photo). Postal code is always 5 digits in Finland. Attached an image where customer has written 6 digit zip-code. Virtuemart does not give any error message when customer is filling in the customer details.

How can I add an error message already to the customer details page if customer has other than 5 digits in the postal code? If not corrected on this page the payment does not go through and therefore the order does not go through. Any ideas how to add the error message to the zip-code field on add customer details page?
Sorry! I don’t know where to post this question…