Hi fran,
Featured image will not be displayed on single page and on pages. You can add featured image on the editor of post/page. We have designed our theme this way and featured image to display in single page is not necessary.
So if you like to have a featured image on single page then it needs code customization.
Go to theme folder -> inc -> structure -> conent-extension.php on line no 225 you will find this code <article class="entry-wrap"> Add below code just after this code
<?php
if( has_post_thumbnail()) { ?>
<figure class="post-featured-image">
<a style="background-image:url('<?php echo esc_url(wp_get_attachment_image_url($attachment_id,'full'))?>');" href="<?php the_permalink(); ?>" title="<?php the_title();?>"></a>
</figure><!-- .post-featured-image -->
<?php } ?>
We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.
Child Theme
Thank you
.
.