Elementor Pro display featured image on section -> style -> image using shortcode

get_the_post_thumbnail() returns the entire img tag, not just the image URL.

global $post;
return get_the_post_thumbnail( $post->ID );

You can also use function wp_get_attachment_url() or wp_get_attachment_thumb_url() to get image URL and insert it into the img tag.

global $post;
$thumbnail_id = get_post_thumbnail_id( $post->ID );
if ( false === $thumbnail_id )
   return '';
$img_url = wp_get_attachment_url( $thumbnail_id );
return '<img src="' . $img_url . '" />';