Try with output buffer:

add_filter('stec_builder_front_elements', function($elements) {
    ob_start();
    wp_editor('', 'stec-builder-element-content-textarea-style', array('textarea_name' => 'description'));
    $editor = ob_get_clean();
    $elements['description'] = array(
        $editor
    );
    return $elements;
});

Try the wp editor js api instead.

Example:

add_filter('stec_builder_front_elements', function($elements) {
    $elements['description'] = array(
        '<div class="stec-wp-editor-container"></div>' // editor placeholder
    );
    return $elements;
});
add_action('stec_after_html', function() {
    // Load editor api js
    wp_enqueue_editor();
    ?>
    <script type="text/javascript">
        (function ($) {
            $.stecExtend(function (m) {
                m.$instance.on('click', '.stec-event-create-form', function () {
                    if (!$(this).hasClass('active')) {
                        var editorContainer = m.glob.options.id + ' .stec-wp-editor-container';
                        wp.editor.initialize(editorContainer);
                    }
                });
            });
        })(window.jQuery);
    </script>
    <?php
});