That is because after upgrading WordPress, the file wp-db.php has been deprecated and replaced with class-wpdb.php.
Solution:
Simply update line 19 in wp_content/db.php as follows:
require_once( ABSPATH . WPINC . ‘/wp-db.php’ );
replace with
require_once( ABSPATH . WPINC . ‘/class-wpdb.php’ );
That should resolve the issue.
