Let's select the sales transaction you need and get this batch email option work for you.
To clarify, are you trying to send a batch email of your sales transactions on the Sales page? If so, let's make sure that we're only selecting invoices and sales receipts from the list. The Send transactions option get grayed out if you include customer payments in that batch email you're attempting to send.
Feel free to see the screenshots below to see what I exactly mean.