Your transaction probably failed because of a system called AVS (Address Verification System). It tries to match the numeric parts of the address you provide in the checkout with the one used while signing up for a card (Curve, in this case).
If the page or app you tried to use accepts UK/European addresses, you should try typing that one in first when entering card details, as the merchant should handle transferring the data through the system correctly. That isn’t always the case, though.
If the merchant didn’t predict non-US card being used in their service, you should try providing the data in the right format yourself, manually. Assuming you signed up for your Curve card as a UK resident, your post code format varies from the ZIP format used in the USA. There is no simple way to determine the ZIP code you should enter, but the following solutions were previously mentioned in this community:
- entering numbers from your UK post code and trailing zeros up to 5 digits,
- entering 99999 for “other state”.
At the same time, the other numeric part of your address will also be checked, quoting Wikipedia:
For example, if the address is 101 Main Street, Highland, CA 92346, in the United States, AVS will check 101 and 92346 .
However this doesn’t have the requirement of being 5 digit long, so it shouldn’t cause a problem. The non-numeric parts of the address don’t matter for card companies (Mastercard) and won’t be checked.
You can find more information about AVS in this thread.