If this option is enabled, Swift will prebuild the cache after it has been cleared. It is recommended to use Optimize Prebuild Only option to achieve the best user experience.
If you can see on the Swift’s dashboard that it is trying to prebuild cache, and load the URLs but the pages aren’t cached, it can be a cache issue, or an error breaks the prebuild process.
Please enable logging in Settings > General and set the loglevel to warning. After that please try to run the prebuild again, and check the Swift’s log.
This option will use WordPress Cron Events. If default WP Cron is disabled, Swift will load wp-cron.php in every minute while the plugin admin dashboard is opened. If the Prebuild process won’t start at all, it can be a WP Cronjob issue. If you are using customized WP Cronjobs, please try to disable them and try it again.