Windows 10 update error 0xa000400, help me fix this, I am unable to update my window?

What OS build are you on?  Hit the Windows key, type winver, hit Enter.
Or Windows key+r, winver, OK

Hello,

I'm your Independent Advisor for today. I know this can be frustrating, but don't worry. I'm here to figure this out for you.

There are many causes to consider when working with Windows Updates, and we need to pinpoint which is causing the problem.

It's possible that a certain app might be causing problem with installing Windows Updates. Please start the PC in a clean boot state to check if there are other background apps which might be causing the issue:

1. Right-click the Windows button > Select Run
2. Type in "msconfig" without the quotes, then click OK.
3. On the General tab, select Selective Startup. Then untick "Load startup items".
4. On the Services tab, click "Hide all Microsoft services".
5. Click "Disable all".
6. On the Startup tab, click "Open Task Manager". A Task Manager window should open.
7. Disable all startup items. You can also choose which startup items to disable, as certain apps and devices might not work properly if the startup item related to it is disabled.
8. Go back to your System Configuration window.
9. Click Apply > OK
10. Click "Restart" when prompted.

Once the PC goes back up, please try to install updates again.

If that doesn't work, you can try renaming the software distribution folders:
1. In the search box on your taskbar, search for cmd
2. Select Command Prompt (with the option Run as Administrator) from the results.
3. At the Command Prompt, type the following commands one at a time. After each, select Enter. Let the command complete before typing the next command.

net stop bits

net stop wuauserv

ren %systemroot%\softwaredistribution softwaredistribution.bak

ren %systemroot%\system32\catroot2 catroot2.bak

net start bits

net start wuauserv

4. Once done, close the Command Prompt window and restart your computer, then try installing the updates.

You can refer to the link below for other steps you can try:
https://support.microsoft.com/en-us/help/408983...

Hope this helps.

Version 1903 (OS Build 18362. 418)

Post back after installing again following Eiji's instructions.

Having the same problem as OP. Build version is 1809, need to update to 1909. tried all of the above solutions, didn't help.