I'm running ATUM 1.6.0 on WordPress 5.2.2. For my browser, I'm using Chrome on Windows.
I cannot seem to add items to a purchase order. As you can see in the attached screengrab, I do have a supplier set and I am positive that they have at least one product assigned to that supplier.
I have entered the supplier, waited. I went ahead and created the purchase order and went back in. I've deleted and recreated a purchase order. No matter what I try, I cannot add anything to a purchase order.