public function new_user(){
   $first_name = $this->security->xss_clean(strip_tags($this->input->post('first_name')));
    $last_name = $this->security->xss_clean(strip_tags($this->input->post('last_name')));
    $profile_id = $this->security->xss_clean(strip_tags($this->input->post('profile_id')));
    $password = $this->security->xss_clean(strip_tags($this->input->post('password')));
    $username = $this->security->xss_clean(strip_tags($this->input->post('username')));
                    $data = array(
                        'first_name' => $first_name,
                        'last_name' => $last_name,
                        'profile_id' => $profile_id,
                        'password' => password_hash($password, PASSWORD_BCRYPT),
                        'username' => $username,
                    );

                    $this->form_validation->set_rules('first_name','first_name','trim|required');
                    $this->form_validation->set_rules('last_name','last_name','trim|required');
                    $this->form_validation->set_rules('username','username','trim|required|');
                    $this->form_validation->set_rules('password', 'password', 'trim|required');
                    $this->form_validation->set_rules('conf_password', 'conf_password', 'matches[password]|required');

                    $this->form_validation->set_message('first_name', 'Please enter First Name');
                    $this->form_validation->set_message('last_name', 'Please enter Last Name');
                    $this->form_validation->set_message('username', 'Please enter Username');
                    $this->form_validation->set_message('password', 'Please enter Password');

                    if ($this->form_validation->run() == FALSE) {
                      $this->json($this->form_validation->error_array());
                    }else{
                      $this->user->signup($data);
                    }
                }

<form id="signup" method="post" onsubmit="return false;">
                        <h2 class="sr-only">New User</h2>
                        <div class="illustration">
                            <i class="icon ion-ios-locked-outline" style="color: rgb(251, 244, 244);"></i>
                        </div>
                        <div class="form-group">
                            <input type="text" name="first_name" id="first_name" placeholder="First Name" class="form-control"
                            required="required" />
                        </div>
                        <div class="form-group">
                            <input type="text" name="last_name" id="last_name" placeholder="Last Name" class="form-control"
                            required="required" />
                        </div>
                        <div class="form-group">
                            <select name="profile_id" id="profile_id" class="form-control" required="required">
                                <option value="">Select a Profile</option>
                                <?php foreach ($profile as $value): ?>
                                    <option value="<?php echo $value['id'];?>">
                                        <?php echo $value['profile']; ?>
                                    </option>
                                <?php endforeach ?>
                            </select>
                        </div>
                        <div class="form-group">
                            <input type="text" name="username" id="username" placeholder="username" class="form-control"
                            required="required" />
                        </div>
                        <div class="form-group">
                            <input type="password" name="password" id="password" placeholder="***********" class="form-control"
                            required="required" />
                        </div>
                        <div class="form-group">
                            <input type="password" name="conf_password" id="conf_password" placeholder="***********" class="form-control"
                            required="required" />
                        </div>
                        <div class="form-group">
                            <button class="btn btn-primary btn-block" type="submit" style="background-color: rgb(41, 54, 78);">Ingresar</button>
                        </div>
                    </form>

{username: "Unable to access an error message corresponding to your field name username.()"}
username
:
"Unable to access an error message corresponding to your field name username.()"

$this->form_validation->set_rules('username','username','trim|required|');

$this->form_validation->set_rules('username','username','trim|required');