You have opened an Outbound Delivery Invoice Request document and you do not see the External Referece.
SAP Business ByDesing
Reproducing the Issue
1. Go to the Customer Invoicing work center
2. Select the Invoice Requests view
3. Search for the Outbound Delivery Invoice Request ABC (ABC represents the ID of the Outbound Delivery)
4. Click the Edit button
5. The External Reference is empty
For the Outbound Delivery Invoice Request, the External Reference which is maintained under the Sales Order document is not supported.
This is the system behavior.
If you would like to have the value maintained under the Sales Order to the Invoice Request, please create an Extension Field and extend it for the Invoice Requests scenario.
Extension Fields Quick Guide
External Reference; Invoice Request; Outbound Delivery Invoice Request; , KBA , AP-CI-CI , Customer Invoice , How To