You've checked the address on the customer card, however, when attempting to calculate tax, you are still receiving error "Unknown country name or code (0)"
- Click on the transaction in GP
- Highlight the line of the transaction and click show detail (circle icon on the far right)
- Review the address at the live level
- Click on the blue arrow on the side by the ship to address
- Update the country field in the address box
- Recalculate tax on the transaction