Recurring Delivery in Adobe campaign is unique as compared to standard delivery. If you are using Recurring delivery Adobe campaign doesn't create a delivery instance until you execute the workflow/delivery. If you want to use the send proof, follow given steps.
1. Enable content approval in delivery; this is used to validate that you are using the correct content.
2. start/execute the workflow/this will generate an instance of delivery under delivery execution folder.
3. Go to delivery execution folder; you will be able to see your delivery, open that delivery from there.
4. Now you can see a button, send proof use that to send a proof, once satisfied approve the content.