When processing an order, you might run into the "Failed to generate label: shipment.References:The billing preference DPT is missing or its value is invalid. 'id' cannot be null." error.

This error relates to a misconfiguration of the Billing Account of the Client you're trying to process the order for.
Feel free to follow the step-by-step guide written below or the video guide, as you prefer:
To solve this issue you will have to:
- Access Clients under the Maintenance menu

- Click on the client related to the order
- Edit the Billing Account (Carrier) related to the order and error

- Select the Carrier Options tab
- Select the reference you want to use in the DPT Reference dropdown or type in a fixed reference
- Press Save to apply the changes

If the rest of the order information is correct, you will be able to Process or Process and Print.