If the purchase order is still open, ensure there are no existing invoices for those receipts:
1. From the purchase order record > select the receipts tab
2. Open the first receipt
3. From the menu bar, select Receipt > Go to > Invoice
4. Any existing invoices would need to be deleted in order to be able to delete the associated receipt.

If the purchase order is closed, it cannot be reopened.
1. Create a purchase order.
2. Post the line item to the date needed.