Usually caused by either corruption or a conversion of a customer, where the Primary and Collections contact info is lost or never entered.

1. Navigate to the Company.
2. Select the Contacts tab.
3. Make sure Primary and Collections contacts are present.
4. If not, Edit Customer and make sure ther's one Primary and Collections contact.
5. Save the Customer.
6. Preview order again.
7. If Primary and Collections contacts are present.
8. Edit the Customer.
9. Select different Contact as the Primary and Collections contact.
10. Save the Customer.
11. Edit the Customer.
12. Select the original Contact as the Primary and Collections contact.
13. Save the Customer.
14. Preview the order again.