I got the same issue. The only thing you can fix this is to ask apple to unlock your Mac and it can be done remotely. My case was still unsolved because i didnt have my original sale receipt so apple just rejected my unlock request. I suspect this is due to Beta version bugs and hoping it will be solved in the next updates of Big Sur

Got the email from apple yesterday and have to provide them additional information for proving my ownership. I may want to wait until the final release of MacOS Big Sur to upgrade myOS if apple rejected my proof of purchase. Hope this bug will be fixed in more stable Big Sur updates.

I got the same issue. My Mac is MBP 2019 13inch Touch Bar with T2 Chip. I already contacted the Apple support team on Monday and still waiting to the security activation team's approval to unlock my Mac. Fortunately, I am still able to get full-access to my Mac for now. I just discovered this issue when I was trying to reset Factory default OS - which somehow enacted the Mac Activation Lock process. Although I am the only user for my Mac, I can not turn-off the activation lock... Following Apple instructions regard to how to turn-off activation lock. I tried multiple times to remove my Mac linked to my apple ID in Find Me, then completely signed-out of my iCloud before starting the reset process but it didn't work. I suspect this issue is nothing to do with my apple ID stuffs, it is about the new MacOS Big Sur Beta bugs or something went wrong with T2 chip. To be real, I think that my case is 50/50 chance of losing my access to my Mac permanently because Apple activation team can deny my proof-of-purchase. I bought my used Mac from unapproved authorized reseller (my sale receipt may be invalid under Apple policy). I am still waiting the result but I am bit worries about the worse case scenario may come. Will keep you update with my case after getting confirmation from Apple. Now I am thinking about the possibility that instead of trying to prove my proof-of-purchase, I might try to convince Apple that I'm the current owner of my Mac by allowing them to get access remotely into my Mac Or just showing them the fact that I tried to turn off activation lock but it didn't work. In addition, my AppleCare+ is still on until 2023 and don't know it would be helpful in my case or not. Any advice would be highly appreciated. I hope anyone got this issue will be solved soon