"You'll need to choose an account if you're using the Category details section. This way, you'll be able to save the purchase order without any errors."

I did not find this as an "option" - Category details section. It is a required field to successfully create and send the PO. Perhaps I need to change a setting somewhere. 

What I did to resolve this pesky error: Added a NEW Category in which I added the Customer/Vendor details.

I did this based on the image I saw on the solution above. It would have been helpful is this step was better stated.