Posts seeking support go in the Help Category. Post moved.
Don't use the add-ons tab, it gets very outdated. Add all plugins yourself.
When downloading a plugin, make sure it's compatible with the server version, put the jar in the plugins folder, and restart your server. Oh, and make sure your server is a Paper server, not Spongeforge or Fabric (mods and plugins are different!)
If it doesn't add your plugin still, please post back with your latest.log.
latest.log can be retrieved by following the steps below:
Navigate to File Manager
Enter the logs folder
At the bottom of this folder, lie a latest.log TXT file
Download that file & upload it here