I can help create a purchase order in your QuickBooks Online (QBO) account.
The Purchase Order feature is only available in the Plus or Advanced version of QBO. If you subscribe for the Simple Start or Essentials version, the said feature isn't available. Thus, you'll want to upgrade your QBO subscription to have the purchase order feature.
However, if you're using the Plus or Advanced version and can't create a purchase order, maybe the feature is turned off. Let's verify it by going to the Account and Settings. Here's how:
In your QBO account, click the Gear icon and select Account and Settings.
Select Expenses from the left menu.
Go to the Purchase orders section, then toggle the Use purchase orders option to turn it on.
On the other hand, both QuickBooks Desktop (QBDT) Pro and QuickBooks Online Plus versions allow you to create a purchase order. The only difference for each version is that you can access QBO anywhere, while in QBDT you can't.