Video not displaying on public post

Hello,

I inserted a vimeo and youtube video using this tutorial : https://en.support.wordpress.com/videos/youtube/ (just pasting the link to the video in the edit-post window).
The video shows up in the edit-post window (in admin) but not on single public view (thee link url appears, taht’s all).

Am I forgetting something ? Should I add something in the single.php template file ?

Thanks in advance.