I am trying to update my sql data through $wpdb, but can't... If anyone can help what's the real issue?
$updated_version = $wpdb->update('wp_visitors', array('views' => $updated), array( '%d' ));
You're missing your where statement. You need to tell
wpdb which row it should be updating, otherwise you're looking to insert rather than update.
wpdb->update( string $table, array $data, array $where, array|string $format = null, array|string $where_format = null );