It's nice to have you in the Community, london2.
Purchase order feature is available in QuickBooks Online Plus and in older QuickBooks Online Edition products.
To enable the purchase order feature:
- Go to the Gear icon.
- Select Account and Settings.
- Choose Expenses.
- Click the edit (pencil) icon in the Purchase orders section.
- Mark the Use purchase orders checkbox.
- Click Save, then Done.
I suggest following this article for more information about purchase order: How to use purchase order.
I'm always here if you need more help.