find posts that don’t have a custom taxonomy

by

Emily’s answer is mostly correct, just that the operator shouldn’t have an underscore in it:

$args = array(
    'post_type' => 'my_post_type',
    'tax_query' => array(
        array(
            'taxonomy' => 'series',
            'operator' => 'NOT EXISTS',
        ),
    ),
);
$query = new WP_Query( $args );