As of the moment creating a column for memo when printing a purchase order is not possible.
I recommend that you add the Memo for Purchase Orders in the Description field so that it can be printed.
You can also search for a third-party provider since this feature is not currently available in QuickBooks Online. Click below to view available third-party applications on our Intuit Marketplace site at https://apps.intuit.com/
I'd suggest browsing these Help Articles. These resources help you efficiently manage your account, income, expenses, and learn more about the software.
If there’s anything else I can help you with, please don't hesitate to let me know by commenting below. I’m always here to assist you further. Take care and stay safe always!