В твоем jar файле нет plugin.yml
В тот раз плагин почему-то скомпилировался с ошибками. Переделал и опять новая ошибка
Could not load 'plugins\TestPlugin.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.IllegalStateException: JavaPlugin requires org.bukkit.plugin.java.PluginClassLoader
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:149) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:394) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:301) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at org.bukkit.craftbukkit.v1_17_R1.CraftServer.loadPlugins(CraftServer.java:394) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at net.minecraft.server.dedicated.DedicatedServer.init(DedicatedServer.java:233) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at net.minecraft.server.MinecraftServer.x(MinecraftServer.java:995) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at net.minecraft.server.MinecraftServer.lambda$0(MinecraftServer.java:303) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at java.lang.Thread.run(Thread.java:831) [?:?]
Caused by: java.lang.IllegalStateException: JavaPlugin requires org.bukkit.plugin.java.PluginClassLoader
at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:50) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at test.main.Testplugin.<init>(Testplugin.java:13) ~[bin/:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:78) ~[?:?]
at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?]
at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499) ~[?:?]
at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128) ~[?:?]
at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:350) ~[?:?]
at java.lang.Class.newInstance(Class.java:642) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:79) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:145) ~[spigot-1.17.1.jar:3212-Spigot-ff89b97-2cdc6b1]
... 7 more