I find youtube links work best when you take the full url from the address page, rather than the share link. Basically just paste the link where you want the video to appear in the post.
You can also use the markdown editor, if you're familiar with it, to post a picture, that is also a link to something such as a youtube video.
Here is a random article i found that seems to cover all the things you can do with it.
https://www.howtogeek.com/448323/what-is-markdown-and-how-do-you-use-it/