Do you mean you uploaded a plugin directly on your server?
Discourse must compile the plugin’s JavaScript assests firstly. You should upload the plugin to a GitHub repository, edit app.yml to add it, then ./launcher rebuild app
Directly uploading the plugin to the plugins folder without compiling will cause the Discourse frontend to not find the compiled file and cause errors.
Also, if you have cloudflare enabled for your server, remember to turn off auto-minify and delete the cache.