I'm having the same problem so I contacted Apple Twitter support. On the app store, it says under Compatibility that my computer is compatible. I even sent them a screenshot (see last attachment). Every time I try to update to Big Sur, I get an error message even though my MacBook Pro (Retina, Mid-2012) meets the specs (and more!) for upgrading.
There is literally no difference in the specs for my MacPro mid-2012 and its 2013 Macbook Pro cousins. Same chip, architecture, RAM. My MacBook Pro is actually faster than low-end 2013 and later models because I upgraded to the fastest everything. This is absolutely infuriating considering I spent over $6,000 so I wouldn't have to buy another computer for a long while. My MacBook is in pristine condition and runs swiftly.
Given I literally just finished paying off this computer, now they're saying I can't update it over some arbitrary and capricious cut-off point, I feel like I'm the proud owner of a brick. I can't help but wonder if this is the same scheme they used with slowing down iPhones in order to force consumers to buy newer models (a lawsuit that they settled).
[Image Edited by Moderator to Remove Personal Information]