Process & could not be started for IDoc number &

What causes this issue?

Process &v2& could not be started for IDoc number &v1&, because in the
partner profile found for the IDoc, no user was entered as the party to
be notified.

System Response

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

How to fix this error?

Please enter a user as the party to be notified in the partner profile.
Other organizational units are not supported for processes.

Error message extract from SAP system. Copyright SAP SE.