Whenever I try to watch a video on YouTube I am faced with an error, no amount of restarts have fixed it and no extensions installed. View attachment 957085
Safari works fine with YouTube on Big Sur for me and has since beta 1; Even in 4KHDR60p which is pretty neat. - Also with nothing special performed - I think the adblocking thing is generally working fine but it's possible YT uses different ad systems across the world and some areas are being flagged as not just an ad but a privacy violation; Safari doesn't per se block ads - it blocks unwanted cross-site privacy violating trackers.
OP; Try enabling the experimental features relating to VP9 - It shouldn't really make any difference, but try, restart Safari and check just for good measure.
Whenever I try to watch a video on YouTube I am faced with an error, no amount of restarts have fixed it and no extensions installed. View attachment 957085