When you clone a multisite network site, and you don’t select the checkbox “Clone the entire network” then WP STAGING will only clone the current website. During that process, it will convert into a single site.
If there are multisite-related plugins on your site that can not run on single sites and if they are developed not so well, the staging site will show a fatal error like: “This plugin only functions on WordPress Multisites” and the website refuses to load.
Due to that, we recommend cloning the entire network. Then the staging site will not be converted into a single site and all network plugins can still do their job as they are intended to do.
If this does not work, and you want to use the single site cloning process, then you need to find out what plugin exactly is causing that error, and you will need to disable it. You can do that by either renaming the plugin folder wp-content/plugins/plugin-name to something else, or you can exclude the plugin on the initial cloning process by excluding its plugin folder.