Billing doc. contains no items

What causes this issue?

The invoice contains no items.

System Response

The system did not post the invoice.

How to fix this error?

Using error/status reporting, check whether errors occurred while the
billing document items were being prepared. Correct the errors and
<DS:TRAN.WPIA>repeat inbound processing </>.
If no errors have been logged, the IDoc may not contain any items. You
can check this by <DS:TRAN.WE21>viewing the relevant IDoc areas</>.

Error message extract from SAP system. Copyright SAP SE.