Entering the details of the vendor's purchase order is all you need to save the transaction, nfresales.
We can try some browser troubleshooting steps to get rid of the error. You can use a private window to see if you'll get the same error when entering a PO. If it works there, go back to your original browser and clear the cache.
QBO is also accessible on other browsers like Google Chrome, Internet Explorer, and Mozilla Firefox. I'd also suggest switching to a different one to create a PO if the error persists.
I've prepared an article for you for the frequently asked questions about purchase orders.
I'll be here if you need a hand. Please let me know if you need anything else.