Assuming the $cats
is receiving values it seems like you’ll want to update the code to:
<?php
$cats = get_field('rel_posts_cat');
$my_query = new WP_Query(array(
'showposts' => '10',
'category__and' => $cats,
));
while ( $my_query->have_posts() ) : $my_query->the_post();
?>
The change is on the 'category__and'
parameter, since $cats should be returning an array assuming you’ve defined the Return Value
to Term ID