Thumbnail image doesn't show up in Shortcode output

A 'WordPress Shortcode' is being developed to display 'YouTube Channel Thumbnail' and 'Latest Playlist Thumbnail' when supplied with 'YouTube Channel ID' as follows.

[yt_insert channel_id=UCbCmjCuTUZos6Inko4u57UQ]

Shortcode implementation is as follows.

function yt_insert_decode( $atts ) {
    $atts = shortcode_atts( array(
        'channel_id' = ''
    ), $atts );

    $api_key = 'YOUTUBE_API_KEY';
    $channel_id = $atts['channel_id'];
    $api_url = 'https://www.googleapis.com/youtube/v3/channels?part=snippet,contentDetailsid=' 
    . urlencode($channel_id) . 'key=' . $api_key;

    $api_response = wp_remote_get($api_url);
    $api_response = json_decode( wp_remote_retrieve_body($api_response) );

    $channel_thumbnail = $api_response-items[0]-snippet-thumbnails-default-url;
    $playlist_id = $api_response-items[0]-contentDetails-relatedPlaylists-uploads;

    $playlist_endpoint = 'https://youtube.googleapis.com/youtube/v3/playlists?part=snippetid='
    . urlencode($playlist_id) . 'key=' . $api_key;

    $api_response = wp_remote_get($playlist_endpoint);
    $api_response = json_decode( wp_remote_retrieve_body($api_response) );

    $playlist_thumbnail = $api_response-items[0]-snippet-thumbnails-default-url;

    $response = 'img src=' . $channel_thumbnail . ' img src=' . $playlist_thumbnail . ' ';
    return $response;

}

add_shortcode( 'yt_insert', 'yt_insert_decode' );

'Channel Thumbnail' DOES NOT show up while 'Playlist Thumbnail' shows up.

How can I change the code so that 'YouTube Channel Thumbnail' shows up in my Shortcode output?