Go to the Gear ⚙ icon, then select Account and Settings.
Navigate to the Expenses tab.
In the Purchase Orders section and click on the Pencil ✎icon.
Mark the Use purchase orders checkbox.
(Optional) Enter titles for up to three custom fields and enter a default message.
Hit Save, then Done.
After reviewing the Settings, let's look at the Purchase Order you need to open. You can link multiple transactions to a single PO. QuickBooks Online automatically closes a PO when all quantities or amounts on the PO are accounted for across all of its linked transactions. You can manually close the PO by changing its status or by manually closing individual lines on it. If you don't need the payment on the PO, then you can Void the Bill or Check so you can break the link to the PO to re-open.
If the issue persists with the Purchase Order not showing as open, then let's try clearing the cache and cookies of the web browser that you're using for QuickBooks Online. This is the best way to handle non-responsive actions in QuickBooks Online because these files can become outdated, causing issues like the one with your Purchase Orders. Here's a link to help you with the process: Clear Web Browser History.