Thanks for reaching out to us. I'm here to help and provide some information about creating Purchase Order (PO) in QuickBooks Desktop.
Are you trying to set up a warning message once you're PO doesn't fill in the complete fields? If so, this feature isn't available in QuickBooks Desktop. I can see how this future would be helpful for you when creating purchase order, I recommend sending feedback directly to our product developers for consideration in the future updates.