Hummingbird can't write into the wp-config.php file

Hummingbird can't write into the wp-config.php file.
- The define('WP_CACHE', true); is added manually to the wp-config.php though the warning still showing up on the
- The wp-content folder permission is 755
- Security plugins deactivated
- The wp-config.php file permissions is 644
- Other caching plugins are deactivated
- wp-content/wphb-cache folder's permissions is set to 755