What’s the problem with my WordPress ajax request

I want to add have a form in my WordPress dashboard that work with ajax but my request response is 400 Bad request

add_action('admin_footer', 'vitrin_admin_ajax');
        function vitrin_admin_ajax() {
        ?>
            <script>
                (function($) {
                    $('.vitrin-private-message-form').submit(function(event) {
                        event.preventDefault();
                        jQuery.ajax({
                            url: '<?php echo admin_url('admin-ajax.php'); ?>',
                            type: 'post',
                            data: {
                                action: 'submit_vitrin_pm',
                                ppmcontent: jQuery('#pmcontent').val(),
                                ppmusers: jQuery('#pmusers').val(),
                            },
                            success: function(data) {
                                // jQuery('#datafetch').html(data);
                                alert(data)
                            }
                        });
                    })
                })(jQuery);
            </script>
        <?php
        }
        ?>

I also add:

add_action('wp_ajax_submit_vitrin_pm', 'submit_vitrin_pm');
add_action('wp_ajax_nopriv_submit_vitrin_pm', 'submit_vitrin_pm');

and

function submit_vitrin_pm() {
    echo 'ssss';
    die();
}