If you and/or your clients are seeing this error when attempting to enter credit card information in the Stripe payment window, it's either because the postal code was not entered correctly, or that it was entered in the wrong area. As shown below, the postal code must be entered in the area highlighted in green.
The expected format of this field varies based on the BIN (Bank Identification Number). For example, if the system identifies the card as being U.S.-issued, the system will expect a 5-digit ZIP (optionally with the “+4” extension); if the card is Canada-issued, it expects a six-character alphanumeric postal code (e.g. K1A 0B1). For countries without postal codes, the field may not be required at all.