I am having process builder error for the auto milestone completion. 
entitlement exit criteria i kept based on case closed date

flow tried to update these records: null. This error occurred: FIELD_INTEGRITY_EXCEPTION: You can’t change the completion date on a milestone that’s already exited an entitlement process.: Completion Date

I am having process builder error for the auto milestone completion. 
entitlement exit criteria i kept based on case closed date

flow tried to update these records: null. This error occurred: FIELD_INTEGRITY_EXCEPTION: You can’t change the completion date on a milestone that’s already exited an entitlement process.: Completion Date