meta_query ‘compare’ => ‘!=’ not working

try this second query

  $c_client_args = array(
                    'post_type' => 'clients_testimonials',
                    'posts_per_page' => 999,
                    'order' => 'ASC',
                    'meta_query'  => array(
                         'relation' => 'OR',
                          array(
                             'key' => 'c_client_feature',
                             'compare' => 'NOT EXISTS' 
                          ),
                          array(
                             'key' => 'c_client_feature',
                             'value' => 'on',
                             'compare' => 'NOT IN' 
                          ),
                        )
                );