Unable to access an error message corresponding to your field name Username.(xss_clean)

The reason for this error is
you didn't loaded the security helper the following is the way enable security helper
with autoload.php in config folder or you can directly load the helper as mentioned last line of this post

And if, despite everything, you really need it, go to application/config/autoload.php :

$autoload['helper'] = array('security');

Or, before your form validation

$this->load->helper('security');