I'll share information about how purchase order works depending on the status of your inventory in QuickBooks Online.
When your inventory is out of stock, the ability to automatically create or trigger a purchase order is unavailable. In this case, you'll manually create and send a purchase order to your vendor.
In the meantime, you can utilize a third-party application to help you with the task. Let me guide you on how.
On the left pane, select Apps.
Go to the Find apps section.
Enter the keyword in thesearch tab.
Select your preferred application.
Click on Get app now.
On the other hand, you can also visit this website to look for one: QuickBooks App Store.
Don't hesitate to visit the Community if you have additional questions regarding your purchase orders or other related concerns in QuickBooks Online. We're always willing to help.