Why this didn’t work if i use get_the_ID(), but works if i pass the numeric id

It is a custom meta box that handles pdf upload. I want to get the URL of the uploaded file. Inside admin, it works but on the homepage, it doesn’t work.

function moin_notifications_get() {
        // Get the 'Profiles' post type
        $args = array(
            'post_type' => 'mmi-notification',
        );
        $loop = new WP_Query($args);
        $file = get_post_meta(the_ID(), 'wp_custom_attachment', true);
            echo '<ul class="moin-notifications">';
        while($loop->have_posts()): $loop->the_post();  
            echo '<li class="moin-note-single"><a href="'. $file['url'] .'">'. get_the_title() . '</a></li>';
        endwhile;
            echo '</ul>';
        wp_reset_query();
    }

Following way it will work.

get_post_meta('6211', 'wp_custom_attachment', true);

Any idea, where I am doing the wrong thing. The problem is with the id.

Edit:
Earlier I used get_the_ID() and that didn’t work.