Captcha doest work on Linux Ubuntu 16.04

Hi guys,
Why the captcha doest work on Ubuntu Linux, I followed exactly like the User guide and even try to watch some videos from youtube, I followed exactly like they did and I also already create the captcha directory on the root directory CI.
But still doest work? What should I do? Pls help.. Thanks a lot

Here is my controller  script:

public function index()
    {
        $this->load->helper('captcha');

        $vals = array(
                    'img_path'        =>    './captcha/',
                    'img_url'        =>    base_url().'captcha/',
                    'expiration'    => 7200,
                    'word_length'    => 8,
                    'font_size'        => 22
        );
        $cap = create_captcha($vals);
        $data['captcha'] = $cap['image'];

        $this->load->view('index', $data);
    }

And this is the views:

                   <div class="form-group">
                        <div class="col-md-5 col-md-offset-2">
                            <?php echo $captcha; ?>
                        </div>
                        <div class="col-md-5">
                            <a href="javascript:;"><i class="glyphicon glyphicon-refresh"></i>REFRESH CAPTCHA</a>
                        </div>
                    </div>

I will appreciate your helps... Thanks