GitHub

How am I supposed to use it?

Exception in thread "main" java.lang.UnsupportedClassVersionError: Launcher has been compiled by a more recent version of the Java Runtime (class file version 60.0), this version of the Java Runtime ...
GitHub

MultiMC Not Launching New Version 1.17, or Many of the Previous Snapshots

Exception in thread "main" java.lang.UnsupportedClassVersionError: net/minecraft/client/main/Main has been compiled by a more recent version of the Java Runtime ...