How to enable lightbox on featured image.

The fix:

Replace

the_post_thumbnail();

with

<?php $featured_img = wp_get_attachment_image_src ( get_post_thumbnail_id ( $post->ID ), 'single-post-thumbnail' );  ?>
<?php if ($featured_img) { ?>
<div class="lightbox-img featured-img-wrap">
<a href="<?php echo $featured_img[0]; ?>" rel="lightbox"><img src="<?php echo $featured_img[0]; ?>" /></a></div>
<?php } ?>

Its not working for me,

I apologize, new to WP and want to add this to my site at grungyape.com.

Plugin installed, just not following your suggestion on adding it to my featured images.

I use a child theme, on what page do I need to make your suggested edit? That may be all I ma missing.

Thanks in advance.

Do it in the file that displays the featured image. It could be your single-post.php. Could be /wp-includes/post-thumbnail-template.php

Link the featured image with this:

<a href="<?php
$feat_image = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
echo $feat_image;
?>" rel="lightbox">

Don’t forget to close the link off.