heywood

As the Help Text states:
Note, this will only work if you have an SSL certificate installed and have HTTPS configured on the server.
This means you actually have to have an SSL Certificate installed and setup for the site before you force HTTPS. Even then it's better to force HTTPS at the web server level. This is an older setting that was mainly used for shared hosts that didn't allow for SSL and web server configurations. These days SSL is standard and pretty much all hosts allow you to configure and force HTTPS at the web server level.
Note:
To revert this setting login to the database, go to the _config table, search for the record where key equals force_https, clear the value column (make it completely empty), and you should be good.
Cheers.