Why can I embed a video from Dailymotion but not Youtube although the code looks right?

I am not a genius coder but it is not the first time I embed a video into a website but this time I am really puzzled with this issue: If I use the piece of embed code given by Youtube (with my own video) I just get an error message and if I use the embed code given by Dailymotion it just work fine. I double-checked the original template (see page here) and it works. My code is absolutely identical but I get the message "Firefox can’t find the file at /embed/RBvoqkGc27U. It either doesn’t exist or the path is incorrect."

Any clue about what is happening here?

view-source:https://template.xtramile.id/TM/Grace/single-portfolio-video.html)