Big sur download getting installation failed installing update iMac 27 inch 2017

Apple is reporting issues with Mac OS update servers.

Apple - Support - System Status

Please try again in a few days, when new versions of Mac OS are released there is a HUGE amount of traffic. Wait until that traffic has died down a bit, perhaps trying late at night or in a few days.

Apple's been having trouble with their servers. At this time the problem is supposed to be resolved.