Not Acceptable! Mod_Security message
"An appropriate representation of the requested resource could not be found on this server. The error was generated by Mod_Security".
If you’re receiving this message when previewing your Revolution Slider from your WP Admin, it means your hosting provider is restricting access to some of WordPress’ native functionality. To solve it, contact your hosting provider to ask them to remove this restriction, or as an alternative, add the slider to an unpublished draft page, and preview the slider from the front-end of your site.