In this case, AFAIK the forum just doesn't support video tags.
It does, but it seems to be picky about the URL, so e.g. it wants the "standard" Youtube URL rather than one of the newer abbreviations.
e.g. this doesn't work
[video:youtube]https://youtu.be/MkatUFHGihg[/video]
but this does
[video:youtube]https://www.youtube.com/watch?v=MkatUFHGihg[/video]