This error message appears because you have selected an invalid country code that is not listed in the country codes table. The value that causes the error is shown in place of the XX. A complete list of all valid country codes can be found here. Please adjust the country code accordingly to book your shipment.
Should you need further assistance with this issue, please do not hesitate to contact us at firstname.lastname@example.org and please send us the reference of the shipment that you're experiencing issues with.