java.lang.NoClassDefFoundError

java.lang.NoClassDefFoundError: net/md_5/bungee/api/plugin/Plugin
	at de.leonhard.xcore.XCoreSpigot.onStart(XCoreSpigot.java:62)
	at de.leonhard.sapphire.Sapphire$Spigot.onEnable(Sapphire.java:297)
	at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:264)
	at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:337)
	at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:403)
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugin(CraftServer.java:381)
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugins(CraftServer.java:330)
	at net.minecraft.server.v1_12_R1.MinecraftServer.t(MinecraftServer.java:422)
	at net.minecraft.server.v1_12_R1.MinecraftServer.l(MinecraftServer.java:383)
	at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:338)
	at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:272)
	at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:545)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.ClassNotFoundException: net.md_5.bungee.api.plugin.Plugin
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:152)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:100)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 13 more