Hello,
QuickBooks Online is not aimed at being simply a "Web-based" version of the desktop product. It's designed to be a different product offering different features needed by customers.
Though you cannot currently mark an invoice as "pending" in QuickBooks Online, what you CAN do is create an estimate template with the header called "Pending Invoice" and then create the estimate and send it to your customer. Once your customer approves it, simply go to the estimate and click "Create Invoice" and QBO will convert that estimate to a fully posting invoice.