Dummheit

@Override
public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {

if(cmd.getName().equalsIngnoreCase("antibookhack")
    if(sender instanceof Player) {
    sender.sendMessage("§6AntiBookHack: §aON");
    }

    else {
    sender.sendMessage("§cYou must be a Player to execute this Command !");
    }
return true;
}