A few days ago a friend of mine had this problem. The update got stuck. In this case the problem was hard disk space. Fortunately I had an external disk with a MacOS installed... I booted the computer using that OS and from there I cleared some space from the internal disk. Then I started the system again and the update worked. It now has Big Sur working fine.
alejandromacia
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The cause of that error in my MBP was free disk space