Fatal error: Uncaught Error: Call to undefined function wp_get_global_stylesheet

Hi, I’m really inexperienced with code, so I need your help!
I have just updated my wordpress and since my site is not visible ( lespervenches.com ). I had several error messages that I managed to correct, but each time I have a new one. I can’t fix this one:

Fatal error: Uncaught Error: Call to undefined function wp_get_global_stylesheet() in /var/www/vhosts/lespervenches.com/public_html/wp-includes/script-loader.php:2322 Stack trace: #0 /var/www/vhosts/lespervenches.com/public_html/wp-includes/class-wp-hook.php(307): wp_enqueue_global_styles() #1 /var/www/vhosts/lespervenches.com/public_html/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters() #2 /var/www/vhosts/lespervenches.com/public_html/wp-includes/plugin.php(474): WP_Hook->do_action() #3 /var/www/vhosts/lespervenches.com/public_html/wp-includes/script-loader.php(2105): do_action() #4 /var/www/vhosts/lespervenches.com/public_html/wp-includes/class-wp-hook.php(307): wp_enqueue_scripts() #5 /var/www/vhosts/lespervenches.com/public_html/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters() #6 /var/www/vhosts/lespervenches.com/public_html/wp-includes/plugin.php(474): WP_Hook->do_action() #7 /var/www/vhosts/lespervenches.com/public_html/wp-includes/general-template.php(3042): do_action() #8 /var/www/vhosts/les in /var/www/vhosts/lespervenches.com/public_html/wp-includes/script-loader.php on line 2322

I have no idea what to do to get my website back. Any idea?
Thanks!