Click on the top left of the screen and select About This Mac.
Now tell us exactly which mac you have and its year.
These are list of macs that can upgrade to macOS Big Sur, if your mac is from 2012 you cannot upgrade
it to macOS Big Sur.
Check compatibility
macOS Big Sur is compatible with the following computers.
MacBook introduced in 2015 or later
MacBook Air introduced in 2013 or later
MacBook Pro introduced in late 2013 or later
Mac mini introduced in 2014 or later
iMac introduced in 2014 or later
iMac Pro
Mac Pro introduced in 2013 or later
View the complete list of compatible computers.
If upgrading from macOS Sierra or later, your Mac needs 35.5GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44.5GB of available storage. To upgrade from OS X Mountain Lion, first upgrade to OS X El Capitan, then upgrade to macOS Big Sur.
There is an error in the App Store where you will see Works with this mac, for any mac.
This error has been in the App Store for over a year.