Well, if you are tech savvy or want to do this manually, then you can follow the step-by-step instructions given
here but as mentioned on the site, skip step #2 and even step #7 initially, do the rest, reboot and try installing the update again to see.
If you get nervous or wanted to use a tool instead,
this tool does just about all of those quickly and properly but please do read
this page with info on this tool and how to run it to know exactly which options you need to focus on.
After installing the tool, make sure to
right-click on its Desktop icon and select "
Run as Administrator"!
This tool is basically a collection of all the native Windows tools and other processes (some mentioned even on this thread) under one script.
You can run them all in the order of the menu but tbh, you really need to focus on the following:
(Skip 1 if you have already created a System Restore Point recently)
Option 2: Resets Windows Update Components
Option 3: Deletes the temp system files
Option 7: Scans image for component store corruption
Option 8: Checks whether the image has been flagged as corrupted
Option 9: Performs repair operations automatically
Option 10: Cleans up the superseded components
Option 11: Deletes any incorrect registry values
Options 13: Reset Microsoft Windows Store
Feel free to run the other check/repair tools between Option 2 and 13 if you like but since your Internet connection is working and you are able to install other updates, resetting WinSock should not be needed, same goes for Disk Checking or System File Checking but it'd not hurt either. After completing all the steps I mentioned, you should restart your computer before trying to install the updates again.
Other than these, there is a chance your system might be infected or there might be a post infection collateral damage. After all, the KB5021234 has mandatory security patches in it as well.
Also, if you do have a Real-Time AV scanner, it'd help to disable it before installing updates (leave it disabled till next reboot).
Hope this helps.