Hi,

It appears the 'font-awesome' hook is already reserved by other plugin using older version of font-awesome.

I've placed this hook in your child-theme functions.php file:

/**
 * Makes sure to load Font Awesome 6 only if there are calendar entries
 */
add_action('admin_enqueue_scripts', 'load_font_awesome_6_style');
add_action('wp_enqueue_scripts', 'load_font_awesome_6_style');
add_filter('style_loader_tag', 'font_awesome_6_loader_tag', 10, 4);
function load_font_awesome_6_style() {
    if (!class_exists('\Stachethemes\Stec\Stachethemes_Event_Calendar')) {
        return;
    }
    $stec        = \Stachethemes\Stec\Stachethemes_Event_Calendar::get_instance();
    $has_entries = $stec->has_calendar_entries();
    $enabled     = \Stachethemes\Stec\Settings::get('misc', 'font_awesome');
    if ($enabled && $has_entries) {
        wp_enqueue_style(
            'font-awesome-6',
            STEC_PLUGIN_URL . 'includes/libs/fontawesome-free-6.4.2-web/css/all.min.css',
            array(),
            STEC_PLUGIN_VERSION
        );
    }
}
function font_awesome_6_loader_tag($tag, $handle, $href, $media) {
    if ($handle === 'font-awesome-6') {
        return sprintf('<link rel="preload" href="%s" as="style" onload="this.rel=\'stylesheet\';" media="%s">', $href, $media);
    }
    return $tag;
}