When processing an order, you might run into a "Failed to generate label: shipment.References:The billing preference PON is missing or its value is invalid. 'id' cannot be null" error.
The error relates to a misconfiguration of the Billing Account of the Client you're trying to process the order.
To solve this issue you will have to:
1. Access Clients under the Maintenance menu
2. Click on the client related to the order
3. Edit the Billing Account (Carrier) related to the order and error
4. Set Reference 1 and Reference 2 either picking an option from the dropdown, or entering a fixed value
5. Press Save to apply the changes