Yea, you can’t use short codes in comments by default with WordPress. Short codes are anything surrounded in those ‘[ ]’ brackets. There are plugins you can install that allow you to use short-codes in posts, products, categories, and other places in WordPress that you aren’t able to by default. However, I don’t know of any that will allow you to put short-codes inside of comments fields since those fields are usually reserved for users to fill out.
There is a way to add your own PHP code to functions.php to add this ability as explained in this article: https://www.isitwp.com/add-shortcode-support-to-comments/
The other option would be to try using the HTML5 tag for videos, . Which would look something like:
<video controls width="250">
<source src="/uploads/flower.webm" type="video/webm">
<source src="/uploads/flower.mp4" type="video/mp4">
<source src="/uploads/flower.ogg" type="video/ogg">
</video>
Although, with the tag option, you may run into compatibility issues with different browsers & Operating Systems. If you do decide to try this option, just make sure that you include a few different video formats like the 3 I included (mp4, webm, & ogg) to increase the chances the video will be compatible and viewable for most people.