I am seeing a strange error in my wordpress site

If you try to visit this site, you will see a fatal error.

`Fatal error: Uncaught Error: Call to undefined function wp_get_server_protocol() in /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php:1412 Stack trace: #0 /web/htdocs/www.dormireacuba.it/home/wp-includes/class-wp.php(733): status_header(200) #1 /web/htdocs/www.dormireacuba.it/home/wp-includes/class-wp.php(753): WP->handle_404() #2 /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php(1291): WP->main(”) #3 /web/htdocs/www.dormireacuba.it/home/wp-blog-header.php(16): wp() #4 /web/htdocs/www.dormireacuba.it/home/index.php(17): require(‘/web/htdocs/www…’) #5 {main} thrown in /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php on line 1412

Fatal error: Uncaught Error: Call to undefined function wp_get_server_protocol() in /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php:1412 Stack trace: #0 /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php(3524): status_header(500) #1 /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php(3471): _default_wp_die_handler(‘<div class=”wp-…’, ‘WordPress &rsaq…’, Array) #2 /web/htdocs/www.dormireacuba.it/home/wp-includes/class-wp-fatal-error-handler.php(233): wp_die(Object(WP_Error), ”, Array) #3 /web/htdocs/www.dormireacuba.it/home/wp-includes/class-wp-fatal-error-handler.php(152): WP_Fatal_Error_Handler->display_default_error_template(Array, Object(WP_Error)) #4 /web/htdocs/www.dormireacuba.it/home/wp-includes/class-wp-fatal-error-handler.php(57): WP_Fatal_Error_Handler->display_error_template(Array, Object(WP_Error)) #5 [internal function]: WP_Fatal_Error_Handler->handle() #6 {main} thrown in /web/htdocs/www.dormireacuba.it/home/wp-includes/functions.php on line 1412`

How to fix it?

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