Please make sure that you have enabled the App, the Coupon field, and the Coupon Pop-up Link in the Voila dashboard.

If it's still not showing up on your cart page/drawer/pop-up, then the App is unable to identify the Checkout button on your cart. You have to manually add the custom button class. If it's still doesn't show up, please start a chat and we will set it up for you in no time.