It happened to me on Big Sur on a 2016 MacBook Pro with touchbar just now (two years later than the post).
In my case the problem was caused by the firewall Lulu that I had installed some time ago. Apparently it was blocking Apple from downloading the update. Uninstalling Lulu removed the problem. Maybe one can fine tune the Application in order to access Apple sites and at the same time keep the app.
Reading previous posts sent me into the right direction, just posting this as a confirmation for anyone finding this problem right now.
paulsally
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