Nothing happen because this:
$this->session->set_flashdata('message', 'HELP!');
Is just setting 'HELP' in the session 'message';
$this->session->set_flashdata('error', 'WOOPS!');
You need to set also some conditions in your view in order to read the session and see the result:
<?php
if($this->session->flashdata('message'))
{
?>
<div class="container" style="padding-top:80px;">
<div class="alert alert-info alert-dismissible" role="alert">
<button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>
<?php echo $this->session->flashdata('message');?>
</div>
</div>
<?php
}elseif($this->session->flashdata('error')){
?>
<div class="container" style="padding-top:80px;">
<div class="alert alert-warning alert-dismissible" role="alert">
<button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>
<?php echo $this->session->flashdata('error');?>
</div>
</div>
<?php
}
?>
As you can see there are here two examples one for the message (success) and one for the error (fail)
EDIT:
As a reaction of your comment I post this here because in the comment wont be readable:
As you say you have already done this:
$autoload['libraries'] = array('database','session');
Do you also have set this in the config?
$config['encryption_key'] = 'DCF564RT9JN761AZX56FR76Rd8hg6s12';
$config['sess_driver'] = 'database';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'ci_session';
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;