PO without PR not allowed

Trigger Event : WHEN-VALIDATE-RECORD
Triggr object : PO_HEADERS
Condition :
(:PO_HEADERS.DOC_TYPE_NAME ='Standard Purchase Order'
and ${item.po_headers.po_header_id.value} is null
and ((:system.block_status = 'NEW' AND :system.record_status = 'NEW')
OR (:system.block_status<> 'QUERY' AND :system.record_status <>'INSERT')
OR (:system.block_status<> 'QUERY' AND :system.record_status <> 'CHANGED'))
)

Actions:
Seq : 10
Type = Message
Message Type = Error
Message Text = 'PO without PR not allowed'


Seq : 20
Type = BuiltIn
Builtin Type = RAISE FORM_TRIGGER_FAILURE