Sales document cannot be saved (see log)

What causes this issue?

Via the custom code in Enhancement Option 'Sales Document Check Before
Save' an abortion of the sales document saving process was triggered.

System Response

The sales document cannot be saved.

How to fix this error?

Check the log for error messages to find out more details on the reason
for the abortion of the document saving process.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.