You are confusing rules with technical limitations. It is clear from your posts that you do not understand the rules.
The thumb-nails appear only to static images. They do not apply to video links.
There is nothing technically wrong. Your post only shows the link because you instructed the code to only show the link.
In fact, what you did was follow the forum rules. We do not want you to post the images of videos in the upper forums -- even though it is technically possible to do so. We do not want the image to show. We just want you to show the link.
You may display the video-images in the Off-Topic Discussion lower forums.
I am willing to show you what you are doing "wrong" but before doing so, you must understand what you should be doing right.
You must acknowledge that we want you to do exactly what you did in the upper forum -- i.e., display ONLY a link.
Do you understand that we do not want you to display the entire image of videos in the upper forum?
You may do so in the Off-Topic Discussion lower forums but not in the upper forums. Is that clear?