I'm recently updating my current snapshot server. It was a 21w19a snapshot world. It's been doing this before I keep updating forward. It's probably trying not to load the world files to prevent the server/game from crashing- but I have a backup so I'm not sure what's wrong. The world was also created on the 21w19a snapshot jar.

Here's the crash log from the console:

[20.05 00:47:08] [Multicraft] Loading server properties
[20.05 00:47:08] [Multicraft] Starting server!
[20.05 00:47:08] [Multicraft] Loaded config for "Special: custom.jar"
[20.05 00:47:08] [Multicraft] Updating eula.txt file
[20.05 00:47:09] [Server] ErrorA JNI error has occurred, please check your installation and try again
[20.05 00:47:09] [Server] Exception in thread "main" java.lang.UnsupportedClassVersionErrornet/minecraft/server/Main has been compiled by a more recent version of the Java Runtime (class file version 60.0), this version of the Java Runtime only recognizes class file versions up to 52.0
[20.05 00:47:09] [Server] at java.lang.ClassLoader.defineClass1(Native Method)
[20.05 00:47:09] [Server] at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
[20.05 00:47:09] [Server] at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
[20.05 00:47:09] [Server] at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
[20.05 00:47:09] [Server] at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
[20.05 00:47:09] [Server] at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
[20.05 00:47:09] [Server] at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
[20.05 00:47:09] [Server] at java.security.AccessController.doPrivileged(Native Method)
[20.05 00:47:09] [Server] at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
[20.05 00:47:09] [Server] at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
[20.05 00:47:09] [Server] at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:355)
[20.05 00:47:09] [Server] at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
[20.05 00:47:09] [Server] at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:601)
[20.05 00:47:09] [Multicraft] Server shut down (starting)

(Sorry if this looks messy. I'll send an image of this crash log just incase this typed out format is a little bit eye straining.)

I don't know what could possibly be causing this. All I can guess is that it has to do something with the 'unrecognizable class file version' statement. However, even then I'm still confused as to what that means. I'm using the proper jar file that the minecraft.net page offered.
Source: https://www.minecraft.net/en-us/article/minecraft-snapshot-21w20a
If you have any idea why this is happening or if there's a similar issue with a solution for this, let me know. This is as much info as I can give.