@wibrst You have to disable Vivaldi’s default ad-blocking to view YouTube videos.
A workaround is the make a quick command for History Back, Delay 100, History Forward. This will skip the initial ad, but not inline ads.
Clearing YouTube cookies, and restarting Vivaldi may make ad-blocking work on YouTube for a week or two, then the message reappears.
There are some other solutions in this thread: Annoying ads on YouTube are back.