Ups…
Change:
php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
to:
while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
Try this:
<?php
// Query Arguments
$ids = get_field('relationship_name', false, false);
$args = array(
'post__in' => $ids,
'orderby' => 'post__in'
);
// The Query
$the_query = new WP_Query( $args);
// Check if the Query returns any posts
if ( $the_query->have_posts() ) {
php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<h1><?php the_field('postField_name'); ?></h1>
<?php endwhile; } ?>