Big Sur update not available

What year model is your iMac?

see > https://support.apple.com/en-us/HT201634

If you have a 2014 or newer iMac, then you can update the macOS to Big Sur 11.1

see > https://support.apple.com/en-us/HT201475

If you have a 2012 or 2013 iMac, then macOS Catalina 10.15 is as high as it can go.

Hi,

Big Sur requires iMac 2014 or later.

https://support.apple.com/kb/sp833?locale=en_US

If your Mac is capable of running Big Sur and you currently have Mojave 10.14 or Catalina 10.15, the upgrade is is System Preferences > Software Update:

I have a MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports) and can't update to Big Sur. I get the "update not available" error. What's the solution?