In most cases, yes the payment policy is no purchase order, no payment. It is important that you put the purchase order number on your invoice to ensure prompt payment. 90% of all Aviva invoices and payments operate this way.
There will be certain suppliers that will not be provided a purchase order as they will be invoicing directly through the Ariba Network on a contract order, you will be told if this applies to you at contract set up and the billing process will also be explained. If you don't know what this is it is unlikely you will be on this payment route.