Are you using a child theme?
Is get_template_directory_uri() function returning the correct URL?
My suggestion is to use get_stylesheet_directory_uri() instead.
wp_enqueue_style( 'validationEngine.jquery', get_stylesheet_directory_uri() . '/assets/css/validationEngine.jquery.css', array(), false, 'screen' );
wp_enqueue_style( 'main', get_stylesheet_directory_uri() . '/assets/css/main.css', array(), false, 'screen' );