Display lightbox image URL ?

Apologies, I don’t think anyone can understand your question. get_post_thumbnail_id() works fine without URLs, it has nothing to do with URLs, only IDs. Are you trying to determine the image URL yourself, or is your code trying to determine the URL? Do you want the URL to display so you can determine the URL, or so anyone can see the URL?

If you just need to know the URL yourself for some reason, you should be able to determine it using your browser’s developer tools. For example, Firefox’s View Page Info tool has a media tab that lists all image URLs used on a page.