Display content from a specific category using cat ID not working

If this is a custom template you would need to use WP-Query to filter out posts for just one category:

<?php
$args = array(
'cat' => '12' // Insert category ID here
);

$query = new WP_Query( $args ); ?>

So your code should read something like this:

<div id="primary" class="content-area"> 
<main id="main" class="site-main" role="main">

<?php
$args = array(
'cat' => '12' // Insert category ID here
);

$query = new WP_Query( $args ); ?>

<?php if ( $query->have_posts() ) : //loop for custom query ?>

 <header>
<h1 class="page-title screen-reader-text"><?php single_post_title(); ?> </h1>   
 </header>

    /* Start the Loop */
   <?php while ( $query->have_posts() ) : $query->the_post(); ?>

        <?php
        /*
         * Include the Post-Format-specific template for the content.
         * If you want to override this in a child theme, then include a file
         * called content-___.php (where ___ is the Post Format name) and that will be used instead.
         */
        get_template_part( 'template-parts/content', get_post_format() );

    endwhile;

    the_posts_pagination();

else :

    get_template_part( 'template-parts/content', 'none' );

endif; ?>

</main><!-- #main -->