I can help you with this. The Purchase Order feature available in QuickBooks Online Plus level accounts is great for letting your vendors know when you need more stock. To start, activate the feature in the Expense settings of your account. Follow these steps.
Click the Gear icon in the upper right corner.
Choose Account and Settings.
Click Expenses from the left menu.
Click the Purchase Orders section to make changes.
Check the box to Use purchase orders.
Click Done to exit out of the settings.
Once it's turned on, you can create them by clicking the Plus (+) icon and then choose Purchase Order. Then you should be good to go. If you're not able to turn on the purchase orders, you're likely on an EasyStart or Essentials level account. You can double check that by going back to the Account and Settings section and choosing Billing and Subscription to see what your account is. If you'd like, the option to upgrade is also right there.
I hope that helps! Our How to use purchase order article has more information about this feature. Take a look and let me know if you have more questions.