Had a similar issue when using iframes in Django,
I could embed most of the videos but some popular music videos would not load and simply display:
Video unavailable.
To fix it had to manually set the Referer header by adding this line to settings.py:
SECURE_REFERRER_POLICY = "no-referrer-when-downgrade"
Turns out Django does not set Referer in request headers by default, and for some videos (e.g. popular music videos that are copyrighted) youtube checks if the Referer has a host name (e.g http://localhost:8000/ in dev or https://examplesite.com), if the Referer is not set or it is simply an IP (e.g. 192.168.1.1:8000) then the video won't play. The "no-referrer-when-downgrade" is 1 of the Django referrer policy settings that adds the referer header except when downgrading from https to http and worked for me.