If you're posting a link to YouTube and there isn't any embedded video appearing in the post, it's possibly because YouTube has started automatically putting a shortener (youtu.be) on its sharing links.
To have a link that will embed properly in a Mastodon etc post, use the full version of the YouTube video's address which begins youtube.com (instead of youtu.be). You can get this full version from the address bar of your web browser when you're watching the video.
@FediTips ohhhhhhh, thank you for this