Minecraft "has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 52.0"

If you get the following error when starting your Minecraft server it's due to you not having the correct Java version installed or being used. Minecraft 1.18, 1.18.1 and higher released in 2021 require Java 17 to run.

INFO Error: A JNI error has occurred, please check your installation and try again
INFO Exception in thread "main" java.lang.UnsupportedClassVersionError: net/minecraft/bundler/Main has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 52.0

If you have a server with us, you can fix this by going to the Startup tab in our control panel and selecting the correct Java version.

If you have a server with another company, you need to contact them to fix it. If you run your own server on your PC or something like a VPS, you need to either uninstall your current Java version and install Java 17, or install Java 17 alongside your existing Java installation and update the system path for it or specify the direct java binary path when starting your Minecraft server.

https://www.oracle.com/java/technologies/javase/jdk17-archive-downloads.html