Use not custom fields in get_posts() meta_query?

No, it doesn’t work like that because meta_query is for custom fields and tax_query is for taxonomies.

But what you could do is run two separate queries to get all of the IDs that qualify, and then use those IDs in a 3rd query with post__in. For example:

$cat_query_args = array(
    'posts_per_page' => -1,
    'cat'            => '63',
);

$meta_query_args = array(
    'posts_per_page' => -1,
    'meta_query' => array(
        array(
            'key' => 'placing',
            'value' => 'nn',
        )
    )
);

$cat_query  = get_posts( $cat_query_args  );
$meta_query = get_posts( $meta_query_args );

$merged_posts = array_merge( $cat_query, $meta_query );

$combined_query_args = array(
    'posts_per_page' => 6,
    'offset'         => 0,
    'post__in'       => $merged_posts
);

$combined_posts = get_posts( $combined_query_args );