Hello and good day, juliayoungmm.

You can customise the Customer Balance Detail report and add the column for the Purchase Order numbers.

Here's how:

1. Go to Reports on the left panel.
2. Type Customer Balance Detail in the search field.
3. Select the Report period, and then click the Customize button.
4. In the Customize report window, click the Rows/Columns drop-down.
5. Select the Change columns link, and put a check mark on the PO numbers box.
6. Hit the Run report button.

I'm also adding these article that you can use for future reference: 

• Customise reports in QuickBooks Online
• Report comparison between QuickBooks Online subscriptions

Feel free to comment on this thread if you need anything else. Take care!