Update Post metafield of specific categories

OK, so a little bit of guessing is involved in that answer, since you’ve posted only parts of your code, but… I assume that your code looks like that:

$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'posts_per_page'   => -1,
    'post_category' => array(14,16,19)
);
$posts = get_posts($args);
foreach ( $posts as $post ) {
    $dealcode = "&deal_code=coupon";
    $urlvalue = get_post_meta( $post->ID, 'metakey', false);
    $urlvalue = $urlvalue . $dealcode;
    update_post_meta( $post->ID, 'metakey', $urlvalue );
}

If so, then the problem lies in this line:

'post_category' => array(14,16,19);

There is no post_category argument for WP_Query…

The correct argument is category__in, so your code should look like this:

$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'posts_per_page'   => -1,
    'category__in' => array(14,16,19)
);
$posts = get_posts($args);
foreach ( $posts as $post ) {
    $dealcode = "&deal_code=coupon";
    $urlvalue = get_post_meta( $post->ID, 'metakey', false);
    $urlvalue = $urlvalue . $dealcode;
    update_post_meta( $post->ID, 'metakey', $urlvalue );
}

PS. Since you only use IDs, there is no point in getting whole posts. It would be much more efficient if you used fields => 'ids' in your query.