After upgrading to the Windows 10 October 2020 update, a bug is not allowing users to perform in-place upgrades using the Microsoft Media Creation Tool (MCT) Windows Latest has learned that this bug will be fixed in updates released on the upcoming December 2020 Patch Tuesday. The easiest way to fix this issue is to roll back to Windows 10 2004 and reinstall the 20H2 update again. To fix the bug, uninstall the update package and restart the computer, uninstalled the update.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-20h2-bug-breaks-in-place-upgrade-feature/