Document cannot be converted: Invalid structure

What causes this issue?

The document to be sent cannot be converted into a sendable format due
to its structure.
E.g. - The document is empty, that is, it contains no objects.
- A version of the document sent previously contains more
or fewer objects.

System Response

Action was terminated.

How to fix this error?

Change or copy the document and resend it.

Error message extract from SAP system. Copyright SAP SE.