Custom route and query

There are 3 issues with your code:

1. The proper syntax of registering your custom REST API route, is as follows:

   register_rest_route( $namespace, $base_URL, $args, $override );
   

   So:

   register_rest_route( 'boss/v1', '/latest-quiz-results', array(
       'methods'  => 'GET',
       'callback' => 'get_latest_quiz_results'
   ) );
   

   Consult the reference for more information.

2. In the get_latest_quiz_results(), $wpdb is not defined because you’re missing global $wpdb; at the top:

   function get_latest_quiz_results() {
       global $wpdb; // Add this.
       ...
   
       return $results;
   }
   

3. Also in get_latest_quiz_results(), you incorrectly called wpdb::prepare() — the second parameter is mandatory, and there needs to be at least one placeholder (e.g. %s). So for example:

   $query = $wpdb->prepare( "SELECT ... WHERE 5PxenC_learndash_user_activity_meta.activity_meta_key = %s", 'pass' );