wp_query args with relation

I think your query is wrong post__not_in only accepts array of post_id’s only.

I think it should be like this.

$onemonth_before = date('Y-m-d', strtotime('-1 month'));
$args = array(
  'tag__not_in' => array(<pass tag_id here>),
  'date_query' => array(
      'before' = $onemonth_before,
      'inclusive' = true,
   ),
);
$query = new WP_Query($args);