Why is my attempt at downloading Big Sur failing?

Solution 1:

Apple servers had a documented outage to the software updates.

• https://www.apple.com/support/systemstatus/

This is why failures happened globally on November 12 during the launch day of Big Sur installations.

• https://support.apple.com/en-us/HT201475 How to upgrade to macOS Big Sur

If you are having issues on or after Friday, November 13 and there isn’t an outage listed in the status page above, and step 4 above doesn’t work follow these steps:

1. Close system preferences
2. Quit any apps that cause internet usage (All web browsers, streaming or games, etc...)
3. Open the Mac App Store and find Big Sur

More details are here on recent changes to this weeks releases - https://eclecticlight.co/2020/11/13/still-getting-errors-trying-to-upgrade-to-big-sur/

If you’ve done anything odd like clearing caches or run tools like Onyx, I recommend you do a safe boot to let Apple clean up properly, then run this command from your terminal.

softwareupdate --fetch-full-installer

If you get any errors, don't worry - wait 5 minutes and try again. (Especially errors about SUPreferenceManager: Failed to set object of class - those do not prevent downloading the latest full installer)

Solution 2:

Big Sur has just been released today. There's thousands of people downloading numerous GBs each all at the same time. Downloads are going to be slow and temperamental until the release settles down a little more.

Your network isn't the bottleneck here; whichever server you're downloading from has limited bandwidth to serve the hundreds of users that may be connected to the same CDN PoP.

Apple doesn't start promoting the update to users for a while after release to make sure all the early adopters have got their download first and the ‘regular’ user base isn't affected.