Why this error is appearing:
This is due to a user deleting the event record or an event unit (the Prices tab on the event record in RE) that is on an unprocessed transaction in the Online Express plugin. 
What needs to be done in the future to prevent this error:
In the future if you need to delete a price type from the event record in The Raiser's Edge, you will want to make sure:
  1. You have downloaded and processed all transactions in Online Express that are associated to the event record/event unit prior to deleting.
  2. You have deleted that price type off of the Online Express form that is associated with the deleted event record/event unit in The Raiser's Edge.

How to address the transactions impacted by this error:
Please contact Support with the following information:
  • When was the last time you were able to successfully process transactions from the affected event form?
  • Was an event record or event unit deleted from the RE database?
    • If an event record was deleted, what event record was deleted? When was the event record deleted?
    • If an event unit was deleted, what unit was deleted and what was the price associated with the unit? When was the event unit deleted?
  • How many transactions in your plugin are linked to this event/unit?
  • A backup copy of your database between the time you were able to successfully process transactions to this event and when you started receiving the error.