Error when trying to upload a Configuration Synchronization file
what you need to do is make few changes on settings.php file

$config_directories = array(

CONFIG_SYNC_DIRECTORY => 'sites/default/files',

);

On production sites, and if your account has permission to do so, it's a good idea to move the configuration sync directory outside of the webroot entirely. As your sync directory should be under version control, this means having your version control repository's root be one level higher than your webroot.  please check out more details at  https://www.drupal.org/docs/8/configuration-management/changing-the-storage-location-of-the-sync-directory