Is it the full site or the mobile site? It'll probably default to the mobile one but there should be a link at the bottom of the page to swap it to the full site. You should then be able to send an invoice.
If it's the website (not the mobile site), then in the drop down menu for "More Actions" for the sold item (extreme right hand side of the page), there should be an option for "Send Invoice" or some other wording.
I understand that eBay usually sends the invoice automatically, at least in my case. You can send/resend the invoice through your Selling management in your MyeBay, which I've had to do in the more recent past for auctions where I did not require immediate payment. Have you contacted the buyer, yet?