A change that was snuck into Drupal version 8.8.0 and wasn't mentioned in the 8.8.0 change notes was a small change to the setting that controls the placement of the configuration directories. The alteration deprecated the $config_directories setting from the settings.php file and move the configuration into the $settings array. You can see the detail behind this change on the Drupal change record.
The fix for this is very easy and only needs a single line of code to change in your settings file.
$config_directories['sync'] = '../config/sync';
$settings['config_sync_directory'] = '../config/sync';
This small change has started tripping me up since I have started to run Drupal 9 upgrades since the previous setting was only deprecated and not removed. Once you upgrade to Drupal 9 you'll immediately see a problem with the configuration directory setting not being present. This can be easily corrected by just swapping over the above variables.