Masalah The action you have requested is not allowed. pada Codeigniter

Gini gan.. Saya udah berhasil Read pada CRUD. Pas saya coba Create nya nggak bisa dan malah muncul The action you have requested is not allowed. Minta tolong bener ini gan. Ini codenya

user_model.php (ini model)

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');

class User_model extends CI_Model {

	//load database
	public function __construct()
	{
		parent::__construct();
		$this->load->database();
	}





	//listing user
	public function listing()
	{
		$this->db->select('*');

		$this->db->from('artikel');

		$query = $this->db->get();
		return $query->result();
	}
//Fungsi Tambah dan validasinya
	public function validation($mode){
		$this->load->library('form_validation'); //meload library validation untuk prses memvalidasi

		if($mode == 'save')
			$this->form_validation->set_rules('judul_artikel','judul_artikel','required|max_lenght[255]');
			$this->form_validation->set_rules('isi_artikel','isi_artikel','required|max_lenght[2500]');
			$this->form_validation->set_rules('jenis_artikel','jenis_artikel','required|max_lenght[20]');
			$this->form_validation->set_rules('keywords','keywords','required|max_lenght[700]');

		if($this->form_validation->run())
			return TRUE;
		else
			return FALSE;

	}

	public function save(){
	    $data = array(
	      		'judul_artikel' => $this->input->post('judul_artikel'),
					'isi_artikel' => $this->input->post('isi_artikel'),
					'status_artikel' => $this->input->post('status_artikel'),
					'jenis_artikel' => $this->input->post('jenis_artikel'),
					'keywords' => $this->input->post('keywords')
	    );

	    $this->db->insert('artikel', $data); // Untuk mengeksekusi perintah insert data
	}

Du.php(Ini file controller)

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Du extends CI_Controller {

	public function __construct(){
		parent::__construct();

		$this->load->model('user_model');

	}

	public function index()
	{
		$user = $this->user_model->listing();
		$data = array('title' => 'Artikel ('.count($user).')',

						'user' => $user,
						'isi'  => 'u/du/list'
					);
		$this->load->view('layout/wrapper',$data,FALSE);
	}

	public function view(){

		return $this->db->get('artikel')->result();

	}

	public function post(){
		$data = array('title' => 'Artikel',
						'isi'  => 'u/du/post'
					);
		$this->load->view('layout/wrapper',$data,FALSE);
	}

	public function tambah(){
    	if($this->input->post('submit')){ // Jika user mengklik tombol submit yang ada di form
	      if($this->user_model->validation("save")){ // Jika validasi sukses atau hasil validasi adalah TRUE
		        $this->user_model->save(); // Panggil fungsi save() yang ada di SiswaModel.php
		        redirect('u/du');
	    	}
      	}
    }
}

post.php(ini file views)

 <form method="post">

		<div class="form-group">
			<label>Judul</label>
			<input class="form-control" type="text" name="judul_artikel" value="<?php echo set_value('judul_artikel'); ?>" required>
		</div>
		<div class="form-group">
			<label>Artikel</label>
			<input class="form-control" type="text" name="isi_artikel" value="<?php echo set_value('isi_artikel'); ?>" required>
		</div>
		<div class="form-group">
			<label>Status</label>
			<input class="form-control" type="text" name="status_artikel" value="<?php echo set_value('status_artikel'); ?>" required>
		</div>
		<div class="form-group">
			<label>Jenis</label>
			<input class="form-control" type="text" name="status_artikel" value="<?php echo set_value('jenis_artikel'); ?>" required>
		</div>

		<div class="form-group">
					<label class="">
						Keywords
					</label>
					<div class="form-control">
						<textarea name="keywords" class="form-control" value="<?php echo set_value('keywords'); ?>" required></textarea>
					</div>
		</div>
		<div class="form-group">

			<input class="btn btn-primary" type="submit" name="submit" value="Simpan">
		</div>


</form>