BattleArena2 3.9.9.15 Error

[13:14:07] [Server thread/INFO]: [BattleArena] Enabling BattleArena v3.9.9.15
[13:14:07] [Server thread/INFO]: [0;31;22m[BattleArena] [0;33;22mv3.9.9.15[0;37;1m enabling![m
[13:14:07] [Server thread/INFO]: [BattleArena] BattleTracker not detected, not tracking wins[m
[13:14:07] [Server thread/INFO]: [BattleArena] Essentials detected. God mode handling activated[m
[13:14:07] [Server thread/ERROR]: Error occurred while enabling BattleArena v3.9.9.15 (Is it up to date?)
java.lang.NoSuchMethodError: org.bukkit.Bukkit.getOnlinePlayers()[Lorg/bukkit/entity/Player;
	at mc.alk.arena.util.ServerUtil.findPlayer(ServerUtil.java:66) ~[?:?]
	at mc.alk.arena.objects.options.DuelOptions.parseOptions(DuelOptions.java:93) ~[?:?]
	at mc.alk.arena.objects.options.DuelOptions.parseOptions(DuelOptions.java:83) ~[?:?]
	at mc.alk.arena.serializers.BAConfigSerializer.parseDefaultOptions(BAConfigSerializer.java:249) ~[?:?]
	at mc.alk.arena.serializers.BAConfigSerializer.loadDefaults(BAConfigSerializer.java:66) ~[?:?]
	at mc.alk.arena.BattleArena.onEnable(BattleArena.java:209) ~[?:?]
	at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:321) ~[spigot.jar:git-Spigot-2130aaa-c8f1723]
	at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:335) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:405) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at org.bukkit.craftbukkit.v1_8_R1.CraftServer.loadPlugin(CraftServer.java:356) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at org.bukkit.craftbukkit.v1_8_R1.CraftServer.enablePlugins(CraftServer.java:316) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at net.minecraft.server.v1_8_R1.MinecraftServer.q(MinecraftServer.java:402) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at net.minecraft.server.v1_8_R1.MinecraftServer.k(MinecraftServer.java:370) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at net.minecraft.server.v1_8_R1.MinecraftServer.a(MinecraftServer.java:325) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at net.minecraft.server.v1_8_R1.DedicatedServer.init(DedicatedServer.java:211) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:494) [spigot.jar:git-Spigot-2130aaa-c8f1723]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_25]
[13:14:07] [Server thread/INFO]: Server permissions file permissions.yml is empty, ignoring it