Relationship & is to an activity which does not exist

What causes this issue?

The MPX file contains relationships to an activity that does not exist.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

INCLUDE 'NA_ALLG_MOEGLICH' OBJECT DOKU ID TX
RESET N1
Correct the relationship in the MPX file.
Call up the MPX file in Microsoft Project (R) and correct the
relationship. Recreate the MPX file.

Error message extract from SAP system. Copyright SAP SE.