error: You cannot create a purchase order and a work order for the same line. How to resolve?

The said error is shown when we are saving a new sales order with assembly item. To resolve this , go to customization>scripts>scripted records

Then undeploy the script which is meant for dropship items and applied to sales order. For dropship items purchase order is created automatically when creating a sales order.Here the value of created PO is setting to dropship. Create a sales order again and then we can see that there is no error issues.