Hi there, enquiries14,

Let me clear things out for you about the purchase order (PO). 

Once you copy the PO to a bill, the system will automatically close the line item. You can still change the status to Open. However, you won't be able to uncheck the Closed checkbox.

You'd need to find the bill and delete it so you can enter a bill again. Let me show you how to do it:

1. Go to Expenses from the left navigation bar, then choose Expenses.
2. Click the Filter icon.
   
3. Select a transaction type, status, date, payee, and category to easily locate the PO.
4. Click Apply.
5. Click the bill to open it, then click More and choose Delete.
   
6. Once done, open the PO and create a bill from there by clicking the Copy to Bill.
   

Learn more about how purchase order works in QuickBooks Online through this article: Create purchase orders in QuickBooks Online.

Should you need anything else, don't hesitate to leave a comment below. I'm always around to answer your questions.