Several people have posted how to fix this error without having to mess with partition sizes, etc. It is called "Repair Install Windows 10 with In-place Upgrade" and it does not delete any files or settings, it just takes a while. Most of the discussion about it was back in May in this thread, around page 42. I copied and pasted one of the replies from there below.
A relatively “simple” solution to the problems caused by the update KB 5034441 which worked for me (see tutorial below)
Having used a copy of a recent Windows 10 installation, downloaded from Microsoft onto a USB, I carried out a “Repair Install Windows 10 with In-place Upgrade” which kept all of my files, apps and settings and then updated and installed KB 5034441 without the need to use Command Prompt, resize partitions, or do anything further. I am neither technically knowledgeable or that competent, so for further information please address any questions to ZEKRI Abdelhafid who’s method this is.
Repair Install Windows 10 with an In-place Upgrade | Tutorials (tenforums.com)
Here's where you can d/l Windows 10: Create Windows 10 Installation Media
About the product key: Find Your Windows Product Key
How to check to see if you have a digital license: Activate Windows
You can also go to Settings > Update & Security > Activation to see if your current copy of Windows has been activated with a digital license linked to your MS account.