If you want to use Java 15 and you want to use SuperiorSkyblock as well, you must download an external scripts engine. The script engine is used for various of things in the plugin, including calculation of levels, for missions and more.
After you downloaded the jar, you need to move it to the modules folder. This process is similar to the process of installing a regular plugin! Simply take the jar and drag it to your modules folder under SuperiorSkyblock's folder.
This is the last step - simply start your server again! The plugin will load the module and set up everything for you! If you get into trouble or something doesn't work for you, make sure you contact us on out discord server.