Yes. At Google we have a saying, “No PO, No Pay”. :-) But very seriously, no work should be performed until a PO is issued. Invoices must reference a corresponding PO number and invoices without a valid PO number will be returned unpaid.
Don’t panic. Simply contact the Google employee who requested your goods or services and ask for your PO number. Keep this number handy! If you’ve already submitted an invoice without the PO number you can add it online using the Provide Missing PO Number tool.