Excluding a Custom Post Type with a specific tag using pre_get_posts

I used tax_query instead of tag__not_in and it works now.

if ( is_post_type_archive( 'event' ) ) {

      $taxquery = array(
        array(
            'taxonomy' => 'event-tag',
            'field' => 'id',
            'terms' => array( 53 ), //the ID of the event tag
            'operator'=> 'NOT IN'
        )
    );

    $query->set('tax_query', $taxquery);
    return;
}