Wordpress Featured Images not appearing using SQL

Question

I am trying to make a query to get the post title, link and images of the post. I have what I need with the following query:

SELECT DISTINCT Users.ID, Users.display_name, PostObject.post_title, PostObject.post_type, Term3.name, Term2.taxonomy FROM `wp_users` AS `Users`
INNER JOIN `wp_3_learndash_user_activity` AS `Learndash` 
ON Users.ID = Learndash.user_id 
INNER JOIN `wp_3_posts` AS `PostObject` 
ON PostObject.ID = Learndash.post_id 
INNER JOIN `wp_3_postmeta` AS `Postmeta` 
ON (PostObject.ID = Postmeta.post_id 
    AND Postmeta.meta_value IS NOT NULL
    AND Postmeta.meta_key = '_thumbnail_id'             
) 
INNER JOIN `wp_3_postmeta` AS `Postmeta2` 
ON (Postmeta.meta_value = Postmeta2.post_id
    AND Postmeta2.meta_key = '_wp_attached_file'
    AND Postmeta2.meta_value IS NOT NULL               
) 
INNER JOIN  `wp_3_term_relationships` AS `Term1` 
ON PostObject.ID = Term1.object_id 
INNER JOIN `wp_3_term_taxonomy` AS `Term2` 
On Term2.term_taxonomy_id = Term1.term_taxonomy_id  
INNER JOIN `wp_3_terms` AS `Term3` 
ON Term3.term_id = Term2.term_id 
WHERE Users.ID = '1' AND PostObject.post_status='publish' GROUP BY Term3.name  
ORDER BY `Term3`.`name` ASC

I am using get_the_post_thumbnail() method to display featured images:

function display_courses(){
    try{
        global $wpdb, $post;
        $current_user = wp_get_current_user();

        $enroll_query = $wpdb->get_results( $wpdb->prepare("SELECT DISTINCT Users.ID, Users.display_name, PostObject.post_title, PostObject.post_type, Term3.name, Term2.taxonomy FROM `wp_users` AS `Users`
INNER JOIN `wp_3_learndash_user_activity` AS `Learndash` 
ON Users.ID = Learndash.user_id 
INNER JOIN `wp_3_posts` AS `PostObject` 
ON PostObject.ID = Learndash.post_id 
INNER JOIN `wp_3_postmeta` AS `Postmeta` 
ON (PostObject.ID = Postmeta.post_id 
    AND Postmeta.meta_value IS NOT NULL
    AND Postmeta.meta_key = '_thumbnail_id'             
) 
INNER JOIN `wp_3_postmeta` AS `Postmeta2` 
ON (Postmeta.meta_value = Postmeta2.post_id
    AND Postmeta2.meta_key = '_wp_attached_file'
    AND Postmeta2.meta_value IS NOT NULL               
) 
INNER JOIN  `wp_3_term_relationships` AS `Term1` 
ON PostObject.ID = Term1.object_id 
INNER JOIN `wp_3_term_taxonomy` AS `Term2` 
On Term2.term_taxonomy_id = Term1.term_taxonomy_id  
INNER JOIN `wp_3_terms` AS `Term3` 
ON Term3.term_id = Term2.term_id 
WHERE Users.ID = '".$current_user->ID."' AND PostObject.post_status='publish' GROUP BY Term3.name  
ORDER BY `Term3`.`name` ASC")); 

        //Check if values is duplicate from any tables
        if ($enroll_query) {
            
            foreach ($enroll_query as $enrollresults) {
                setup_postdata($enrollresults);
                echo $enrollresults->post_title;
                echo get_the_post_thumbnail($enrollresults->ID, 'thumbnail');
                
            }
            wp_reset_postdata();
        }
    }
    catch(Exception $e)
    {
        echo 'Error writing to database: ',  $e->getMessage(), "\n";
    }

}

However when I run my SQL query, no thumbnail images are appearing on the front page even I call the table column meta_key field _thumbnail_id. Is there a way to show the thumbnail on the site?