There are two possible scenarios on what needs to be done to resolve the error:
1. ISO code on the country that is selected on the order is incorrect and needs to be amended. Please check from Linnworks.net > Settings > Countries list if the correct ISO-2 has been entered as the country code.
2. Selected service cannot be used for the shipment, and you need to choose an alternative one. This may happen for example when you try using DPD Classic (Road) service to an island destination like Cyprus. In this case DPD Air service needs to be used instead.