lightbox in self made template

So i installed the new version of wordpress ( clean install ).
I made my “own” template through this tutorial ( https://webdesignerwall.com/tutorials/building-custom-wordpress-theme )

Everything works as it should but i can’t get lightbox to work, the problem is it doesn’t add the rel=”lightbox”. I tried almost 6 differen’t plugins.

I already did add <?php wp_footer(); ?> to my footer.php just before the </body> tag.

Can anyone help me with this?

Kind Regards!