When u want to add the plugin it needs to be like this: let's say your server is 1.12.2 for example then u Need to look that the plugin would be like this: pluginname and then at the right of the plugins name it is a green title with the version of your Minecraft server. EXAMPLE: AWESOMEPLUGIN. (1.8✓)
If there isn'ta green text next to the plugin with your Minecraft server version then that plugin will not work.