Twitter card issue with my site


I've been unable to get twitter cards working with my site - either with SEO Yoast nor with Twitter Cards Meta plugin. When I went to Twitter's Card Validator I get 'Unable to render Card Preview'. In the info section it shows the following:

INFO: Page fetched successfully
INFO: 20 metatags were found
INFO: twitter:card = summary tag found
WARN: Not whitelisted

So I decided to add the code directly in a child theme I have using this walkthrough. This is the code I added in my functions.php file:

add_action('wp_head', 'add_twitter_cards');

function add_twitter_cards() {
    if(is_single()) {
        $tc_url    = get_permalink();
    $tc_title  = get_the_title();
    $tc_description   = get_the_excerpt();
    $tc_image = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), full );
    $tc_image_thumb  = $tc_image[0];
    $tc_author   = str_replace('@', '', get_the_author_meta('twitter'));
    <meta name="twitter:card" value="summary"/>
    <meta name="twitter:site" value=“@david_hammy” />
    <meta name="twitter:title" value="<?php echo $tc_title; ?>" />
    <meta name="twitter:description" value="<?php echo $tc_description; ?>" />
    <meta name="twitter:url" value="<?php echo $tc_url; ?>" />
        <?php if($tc_image) { ?>
      <meta name="twitter:image" value="<?php echo $tc_image_thumb; ?>" />
        <?php } if($tc_author) { ?>
        <meta name="twitter:creator" value="@<?php echo $tc_author; ?>" />

I still get the same result in the Twitter validation page and when I send for approval Twitter responses with the following:

The URL you provided for .... to use the summary card did not validate. Please make sure the page contains valid metadata, then reapply for a Twitter card.

Any thoughts would be much appreciated! This is my site. and you can check the meta data in the header.