This error indicates that the Java running is too old for the server or plugin class it is trying to load. Find out which component the class in the first error comes from.
Step by step
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Read the first line
UnsupportedClassVersionErrorin the boot log. Determine whether the error occurs in the server jar or in a named plugin. -
Type
java -versionin the environment where you run the initialization file. The Java selection on the panel may not be the same as the terminal version on the computer. -
Select supported Java for the server. If you only want one plugin with newer Java, you can also consider the plugin version that is compatible with the same version of Minecraft and Java.
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Restart and verify that the error disappears and the server completes booting. If it's a plugin, check the installation line as well.
Keep in mind
Changing the jar name or downloading the file again will not upgrade the old Java runtime.