Why is my attempt at downloading Big Sur failing?
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:

Close system preferences
Quit any apps that cause internet usage (All web browsers, streaming or games, etc...)
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)