Dear Kaushal Bajaj,
wp_get_active_and_valid_themes() is a private core function, which means plugins and themes aren’t supposed to use it. It’s also defined on wp-load.php. This most likely indicates one of two things.
1) Your WP Installation isn’t valid. Reinstall the most recent version of WordPress and it should fix itself.
2) If you’re attempting to use WordPress functions outside of WordPress (for example, in a custom PHP file), you’re not properly loading in wp-load.php with require_once( ‘path/to/your/wp-load.php’ );