Site seems to miss the "private" wrapper, how can I fix it? I can not enable the private folder getting the warning as below:

Warning: file_exists(): Unable to find the wrapper "private" - 
did you forget to enable it when you configured PHP? in file_save_htaccess() 
(line 365 of core/includes/file.inc).


It's Drupal 8.6.12, multisite config, other sites use private folders.

How can I fix it ?
Here are the necessary steps:


Create a directory on the server. This is typically done via SSH. For instance assuming Apache under Ubuntu, the following command can be used: sudo -u www-data mkdir -p /var/www/html/web/sites/default/files/private
Tell Drupal about your directory by editing your settings.php file. You need to have a line like the following:

 $settings['file_private_path'] = '/var/www/html/web/sites/default/files/private';

Important: clear the Drupal cache, for example with drush cr or by going to the page /admin/config/development/performance in your browser.
In the browser's address bar, go to /admin/config/media/file-system. You should see "Private local files" in the "Default download method" option. If not, then please visit the /admin/reports/status page to see any error messages.  Make sure that PHP can write in the directory. Often, for this to work, the directory has to be writable for the www-data user.


For more documentation, go to https://www.drupal.org/docs/8/core/modules/file/overview#private-file-system.