I think the problem is your variable: $post_ID. Maybe try calling the global $post object and then using $post->ID like so:
<?php // get WP global post object
global $post;
// you could add a query here to limit your results
get_fields($post->ID);
// Also if you are grabbing multiple fields you might need to loop through them
$fields = get_fields($post->ID);
foreach ($fields as $field) {
// do something with each post
}
Not sure if that is exactly what you’re looking for.