Diagnosis
You want to maintain plant maintenance data for an item which is notrelevant to plant maintenance (eg. PM assembly).
However, PM data can only be maintained is the item status of the itemin question contains the Plant maintenance indicator.

Procedure
Check your entries.
You have two options:
Set the Plant maintenance indicator in the item status (--> itemrelevant to plant maintenance).
Remove your entries in the fields which are relevant to plantmaintenance.