Just a general sanity check, make sure you’re using a server type that supports plugins, such as Paper or Spigot.
Also, be aware that due to EssnentialsX’s download setup, the service that AMP uses to install plugins cannot download it properly, you’ll need to go to their site and download it manually, the upload it to AMP’s web file manager.
If neither of those solutions work, provide the server’s latest.log, as that’ll say why the plugin isn’t loading.