Omono has no separate invoicing screen. An invoice is a shipped dispatch, so you find it, print it and resend it from the sales order.
Find the order. On the Sales Orders screen, open Advanced and type the number only (1234, not INV-1234) into Invoice #. Tick Complete in Status and set Division to All, because those filters still apply. Or type the number, without INV-, into Quick Search (Ctrl+Q).
On the sales order page, find the dispatch whose header shows that Invoice #.
To view or print it, click the page icon on the dispatch header to open the Documents form, then click Invoice. Omono downloads the PDF; open it to view or print.
To resend it, click the mail icon on the dispatch. In the Dispatch Email, tick Invoice as an attachment and send.
Invoice appears only once the dispatch has shipped. Of the documents Omono creates, only the delivery note and the certificate of conformity can be pre-ticked, in the Auto Attach column under Settings → Templates → Dispatch Email, so tick Invoice yourself each time.
For how the invoice is created, its due date, what prints on it and automatic invoice emails, see Invoices and proforma invoices. For recipients and attachments, see Sending documents by email.