Can't set customer form error

You need to validate the login with a callback.
https://www.codeigniter.com/user_guide/l...on-methods

You can modify mine if you want.

PHP Code:

public function login()
{
    $this->load->helper('form');
    $this->load->library('form_validation');
    
    $validation_rules = array(
        array(
            'field' => 'username',
            'label' => lang('login_email'),
            'rules' => array(
                'trim',
                'valid_email'
            )
        ),
        array(
            'field' => 'password',
            'label' => lang('login_password'),
            'rules' => array(
                'trim',
                'required',
                array('validate_auth', array( $this, '_validate_auth' ) )
            )
        )
    );
    
    $this->form_validation->set_rules( $validation_rules );
    
    if( $this->form_validation->run() === TRUE )
    {
        return TRUE;
    }
    return FALSE;
}

// --------------------------------------------------------------

public function _validate_auth()
{
    $user_email        = $this->input->post('username');
    $user_password    = $this->input->post('password');
    
    if( empty($user_email) OR empty($user_password) )
    {
        $this->form_validation->set_message('validate_auth', lang('error_missing_fields'));                
        return FALSE;
    }
    
    if( $auth_data = $this->auth_model->get_auth_data( $user_email ) )
    {
        if( $auth_data->banned === '1' )
        {
            $this->form_validation->set_message('validate_auth', lang('error_username_password'));
            return FALSE;
        }
        if( ! $this->check_password( $auth_data->passwd, $user_password ) )
        {
            $this->form_validation->set_message('validate_auth', lang('error_username_password'));
        }
        else
        {
            // Setup redirection if redirect required
            $this->redirect_after_login();
                    
            // Set session cookie and remember me
            $this->maintain_state( $auth_data );
            
            // Send the auth data back to the controller
            return TRUE;
        }
    }
    else
    {
        $this->form_validation->set_message('validate_auth', lang('error_username_password'));
    }
    
    return FALSE;
}