Invoice document &1 / &2 blocked for processing

CAUSE&
You cannot process the required invoice document, as it is blocked by
another application or user.

System Response

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

WHAT_TO_DO&
Make sure that you are not processing the invoice document in another
session. If you are, exit the application in the other session.
The invoice document may also be blocked by another user or be being
processed by the Invoice Verification in the background program.

Error message extract from SAP system. Copyright SAP SE.