Clear out your macOS system caches with Onyx, reboot your machine and try downloading it again.
In addition to other answers here, I will explain what I experienced after Apple’s screw ups today.
- I saw that Big Sur was released around 1:30pm EST.
- I started the download and it was slow as molasses; telling me it will take 12 hours to 1 day to download.
- Despite that abysmal download speed estimate, I let the download run in the background as I did other things.
- Then — maybe at around 2:00pm or so — my system became slow to unusable due to the issues with Gatekeeper/OCSP and application signing.
- I stopped the Big Sur download and attempted to get my system back to a usable state by force rebooting and such.
- At one point I even ran Onyx to deep clean all of my Mac’s caches in an attempt to get my system to work not knowing about the Gatekeeper/OCSP stuff.
- Finally — after some time passed during the afternoon — my system was usable again due to Apple fixing their Gatekeeper/OCSP remote server. And I was able to get back to work.
- But later in the day — around now; 7:00pm to 8:00pm EST — I continued to get those “Installation failed: An error occurred while installing selected updates.” messages seemingly endlessly.
- Frustrated again, I ran Onyx again and lo and behold… I am now downloading Big Sur; 3.25GB of 12.18GB so far with a 2 hour estimate for download.
So while I am willing to accept the idea that Apple’s servers are overwhelmed, I don’t find that to be a clear answer here. Between the Gatekeeper/OCSP issues as well as my need to run Onyx to clear caches to get the download to start again, this all points to something else happening.
I mean at this point the canned excuse of Apple’s servers being overwhelmed to handle a planned software update seems — honestly — to be silly. Apple can cleanly get iOS updates out with little issue and streams audio and HD videos all the time. But somehow core OS updates fail like this? Nah, Apple screwed up
And I am not the only one to share the opinion that the scale and demand for Big Sur cannot really be considered a factor in this; here is a take on AppleInsider about the issues that happened on November 12, 2020; bold emphasis is mine:
It wasn't about demand
It's also a little too easy to blame the problems on just how many people were trying to download macOS Big Sur. This was a failing, it isn't an excuse to claim Big Sur is popular.
Again, there aren't many companies that can push out an OS update to so many users, but this is was actually one of Apple's smaller cases.
True, macOS Big Sur was a very large file to download, but according to Apple's last unit sales volume data from a few years ago, there are at least 20 iPhone users to ever one Mac owner - and this ratio has surely only increased with time. So iOS 14, for example, was a far bigger deal to distribute from a volume of data perspective.
Apple can do OS distributions at large scale. Apple has now done this many, many times. And, it does it with popular media as well — the download crushes from a new Disney movie are fairly incredible, we understand, with 4k movies of similar size as Thursday's Big Sur download.
But at least there are online communities — such as this one — where we can share experiences to move forward.