While mapping/editing an AEP segment in AAM, the following error occurs:
There were errors, trying to save the segment.
Policy enforcement service error: UPSERT forbidden. Upsert will result in parent dangling reference.
This could happen at the time of enablement from AEP to AAM connector when backfill is going on but that has a very short window.
Retrying after some 5-10 minutes gap usually result in successful action.