You have installed a SSL certificate into your site (server) however you aren't getting the correct https protocol on the checkout even after you have clicked forced SSL on checkout via your back-end admin area.
This happens to a lot of sites generally because of a misconfiguration inside of your theme. Here are the steps in order to track down your issue.
- Make sure you go to your website without being logged in
- Proceed through the checkout as a standard user would and make sure you have added something to your cart
- Once you have added your product and are now at the checkout page, you will need to review the page - or inspect as Safari would say
- Look for anything in the console that says something like "[Warning] The page at https://yourdomain.com/checkout/ displayed insecure content from http://yourdomain.com/wp-content/uploads/2014/01/sidebar_logo2.jpg
The above is suggesting the the file, in this case the logo, isn't being served up via https and is a no red flag to SSL. As the SSL page must be entirely under https or it will break and revert back to http, which you don't want.
To fix the above example, you should use a relative URL which could look like either of the following;
- /wp-content/uploads/2014/01/sidebar_logo2.jpg, or