A plugin needs a newer Java than the server runs
Applies to Paper, Vanilla.
What it looks like in your log
Could not load 'plugins/MyPlugin.jar' in folder 'plugins': java.lang.UnsupportedClassVersionError: class file version 65.0
What causes it
A plugin was compiled for a newer Java version than the one running this server.
How to fix it
Switch the server to the Java version the plugin needs, or use an older build of the plugin.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log