This error message is indicating that events cannot be deleted as they have been placed on the itinerary of a group sales reservation. To delete the event, first delete the event from the itinerary.

To find a list of orders this event is attached to:
  1. Go to Analysis > Query
  2. Add a new Ad-hoc query
  3. Select Sales Orders as the source view
  4. Select the field Sales Order Item\Sales Order Item Ticket\Program Events:Event Record
  5. Choose the filter, Is equal to, and search for the appropriate event record
  6. The results will then display a list of associated orders after processing the query