Creating invoices and purchase orders in QuickBooks Online is made so simple with the user-friendly interface. I know how important it is to be able to easily create these forms, and I'd be happy to explain how that works in QBO.
You're unable to create a purchase order from an invoice, as the purchase order is a non-posting 'expense' transaction (something you create to reflect what you're ordering from your suppliers), and an invoice is a sales transaction (what you're going to bill your customer for).