@UnexpectedWebsites had you modified the code in a previous version? Automatic Youtube Video Posts only stores the YouTube video's ID instead of an embed code or URL, which means the plugin doesn't know it's from YouTube. You'll have to change the line
$markup = get_post_meta( $post_id, $video_key, true );
$markup = 'http://youtube.com/watch?v=' . get_post_meta( $post_id, $video_key, true );
and set the custom field value to
_tern_wp_youtube_video. Let me know if that doesn't work.
@microo12345 you should only enter something in the custom field setting if your video is being stored in a custom field instead of the regular post content