SUS Invoice against Purchase Order

Hi Experts,

How to create one SUS Invoice for one whole purchase order directly?

Let us say – there are two line items in a purchase order. In SAP a GR is created for the material & a Confirmation is created from SUS and released
in ECC. Supplier wants to create only one invoice for the whole of this purchase order. 

When Supplier open the “Create Invoice”, he has an option to create Invoice “For a purchase order” which lists the Purchase Order (which has GR and SES created) but when “Create Invoice” button is clicked–  SUS shows a success message that “No suitable or open items found”?

Any idea on what is wrong?

Thanks