Activity posting document & & no longer exists

What causes this issue?

You want to reverse a backflush that was used to enter production
activities. The documents to be reversed must be imported.

System Response

The original activity document & & no longer exists.

How to fix this error?

Test whether the activity documents have been archived/reorganized.
Without the corresponding activity document, no document-related
reversal can be executed. If the activity document to be reversed
cannot be retrieved, you can execute a document-neutral reversal.

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.