Can you try with below?
select OrderHeader.OrderID,OrderHeader.CustomerID,OrderHeader.Date,OrderPostion.PosID,OrderPostion.ItemID,OrderPostion.Quantity from OrderHeader JOIN OrderPostion on
OrderHeader.Index = OrderPostion.Index and OrderPostion.Index in
(Select StatusProtocol.Index from StatusProtocol where SStatusProtocol.status='Ready', StatusProtocol.Type = 'Order', StatusProtocol.SenderSys = 'ExternApp')