Why is the checkout button not showing on my site?

PRESSERO > *Pressero FAQs
If you have an Advanced Payment Method enabled for your site, such as Paypal Pro or Authorize.net, please make sure that your site is running on https. Check the browser address bar or retype the address with a prefix of https:// to check if the site is running secure. If the site is loading only on http, the checkout button will not be shown when a user attempts to checkout an order.
 
To set a Pressero site to load securely on https, use the settings in the Domains window under Sites > sitename > Domains. Please refer to the article Domains tab in Settings for more information.