It was sealed and not used by anyone else. Also, I have only two users: My main user with "Admin" access and a Guest, that is turned off.
There is also the same issue with my iMac 2019
I found that some custom profiles might block those updates, it is a must for my work to have them, I am not sure how can I edit it and which flags to look at to be sure that this is the cause.
Now there is definitely one option that looks strange but logically it should not be the issue:
macOS SoftwareUpdate: Automatically install macOS updates - False
So I don't see the update to the new macOS button in software updates settings, instead, I try to do it from the App store - and it fails with the error above.
And I confirm that I can update apps with no problem, but not the macOS.
Maybe this is what you could shine some light on?
Thank you!