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:

1. Go to the Gear icon. 
2. Select Account and Settings.
3. Choose Expenses.
4. Click the edit (pencil) icon in the Purchase orders section.
5. Mark the Use purchase orders checkbox.
6. 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.