penyebab image tidak dapat di upload di codeigniter ?

permasalahanya saya tidak bisa upload gambar melalui form jadi yang di triger adalah kondisi di bawah :

if(!$this->upload->do_upload('userfile')){
                // $error = array('error' => $this->upload->display_errors());
                // $post_image = 'noimage.jpg';
                die('gagal');
            }

ini code controller saya :

public function create(){
        $data['title'] = "Form Create";
        $data['categories'] = $this->Post_model->get_categories();
        $this->form_validation->set_rules('title','Title','required');
        $this->form_validation->set_rules('body','Body','required');

        if($this->form_validation->run() === FALSE){
            $this->load->view('templates/header');
            $this->load->view('posts/create', $data);
            $this->load->view('templates/footer');
        }else{

            $config['upload_path'] = './assets/images/posts';
            $config['allowed_types'] = 'gif|jpg|jpeg';
            $config['max_size'] = 2048;
            $config['max_width'] = 1000;
            $config['max_height'] = 1000;

            $this->load->library('upload', $config);

            if(!$this->upload->do_upload('userfile')){
                // $error = array('error' => $this->upload->display_errors());
                // $post_image = 'noimage.jpg';
                die('gagal');
            }else{
                $data = array('upload_data' => $this->upload->data());
                $post_image = $_FILES['userfile']['name'];
            }
            $this->load->library('upload', $config);
            $this->Post_model->create_post($post_image);
            redirect('posts');
        }
    }

form create saya, saya tambahkan input file :

<h2 class="text-center" ><?= $title ?></h2>
<?php echo validation_errors(); ?>
<?php echo form_open_multipart('posts/create'); ?>
    <div class="form-group">
      <label for="">Post Title</label>
      <input type="text" class="form-control" name ="title" placeholder="Post Title :">
    </div>
    <div class="form-group">
      <label for="exampleTextarea">Body :</label>
      <textarea class="form-control" id="exampleTextarea" rows="3" name="body" placeholder="Add Body :"></textarea>
    </div>
    <div class="form-group">
      <label for="exampleSelect1">Example select</label>
      <select class="form-control" name="category_id">
      <?php foreach($categories as $cat) : ?>
        <option value="<?= $cat['id']?>"><?= $cat['name']?></option>
      <?php endforeach; ?>
      </select>
    </div>
    <div class="form-group">
      <label for="upload Image">Upload Image</label><br>
      <input type="file" name="userfile" size="20">
    </div>
    <button type="submit" class="btn btn-primary">Submit</button>
<?php echo form_close(); ?>