If you’re trying to download today’s upgrade to Big Sur, I’m afraid that Apple’s servers – Big Survers maybe? – have gone on pause in many parts of the world at present. As a result, your attempts may fail. I’m afraid that I’m giving up for the night now, as both the systems I want to upgrade are eliciting the same failures repeatedly.
Can things get better on Friday 13th?
Apple has apparently fixed that service, but for the time being the Big Sur upgrade is likely to remain unavailable. It’s hoped it will be restored some time in the coming hours.
(2330 UTC 12 November 2020; sorry, I’m going to bed now!)