post__not_in
accepts an array so change
<?php $postid = get_the_ID(); // capture the id ?>
to
<?php $postid[] = get_the_ID(); // capture the id ?>
and you’ll be fine.
post__not_in
accepts an array so change
<?php $postid = get_the_ID(); // capture the id ?>
to
<?php $postid[] = get_the_ID(); // capture the id ?>
and you’ll be fine.