This occurs when the default delivery method requires fields that the patron does not have.  For example, if the delivery method requires a phone number and you are either not selecting a patron, or the patron does not have a phone number, the error will occur.  

Use the following steps to correct the error: 
  1. In Tickets, Delivery Methods, highlight the method and click Edit. 
  2. Unmark the checkboxes to remove the required fields. 
  3. Add the payment on the order.