@miguelaichon – that’s a key bit of information because that means that it’s not finding ‘attachment_field’ correctly.
So you’d need to check some elementary things: Are you testing this on a post that in fact has content in that sub field? Is it named correctly?
Also, if you are having to specify the post ID in all your other fields, perhaps you need to use it in the sub field too?… IE $valueField = get_sub_field('attachment_name', $post->ID);
