Big Sur Software Upgrade
Why does my iMac not upgrade to Big Sur?
iMac 27″, macOS 10.15
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Hi, welcome to the Apple Users Community.
Without know the age of your iMac it isn't possible to tell you that but check this link for a list of compatible units.
https://support.apple.com/en-gb/HT201475
Also it may not be the wisest option to upgrade just yet if you use any particular software packages that may not have made necessary updates to their systems to work correctly with Big Sur. Please make sure to confirm this isn't something you will encounter before making the jump up. And also if you do decide to follow the instructions for making a system backup in case you need to revert back.
Best of luck