new update

You're checking for updates, which are minor changes and not expected to disrupt applications. You're asking for what Apple calls an upgrade, which are larger changes, and can and various will break some apps. In this case, upgrading from 10.12 Sierra to 11 Big Sur will break every 32-bit app still in use; all of those will need to be updated, upgraded, replaced, or retired. This includes 32-bit drivers for third-party printers and scanners. (To find your 32-bit apps, see the Go64 tool.)

For example, Microsoft Office 2011 and earlier are 32-bit apps and will not work on Catalina and later. You will need Office 2016 (retired), Office 2019 (current), Office 2021 (soon), an Office 365 subscription, or a switch to LibreOffice or Apple iWork Pages, Numbers, and Keynote, or other app, if you need access to Office documents.

Have a complete and backup before the upgrade.

Big Sur, tap or click or details: https://www.apple.com/macos/how-to-upgrade/

Older versions, tap or click: https://support.apple.com/en-us/HT211683

Your specific iMac 21.5" model will have a range of upgrades. To upgrade to Big Sur, it must be iMac 21.5" 2014 or newer. Older iMac models can upgrade to older versions of macOS. Yours can upgrade to at least High Sierra 10.13, and possibly newer depending on the iMac model year.

The operating system your Mac can run depends on its year and model. You may not be able to run Big Sur. Incidentally, that is an upgrade, not an update. To see what your Mac is, open About This Mac which is below the Apple logo. Then the Apple web sit has the information as to which models can accept which upgrades. You can then find them at the Apple App store.

I assume that you are currently Running the earlier version of Big Sur. I would simply try again later.