Windows update could not be installed: error 2147942487

So the update to 1803 did not finish? Or did it and you are having these problems on 1803?

You may need to update the BIOS (and Chipset and Video) on that machine as Windows Updates need updated BIOS in some cases I have seen here.

I updated one machine from 1607 to 1709 with a BIOS update in between.

Also make sure there is no legacy software, particularly Anti Virus. Maybe remove (uninstall) your Anti Virus and try updating again.

Event viewer gave me a message that 1803 completed successfully but the version is still at 1603. When I went to check for updates again it said that it was already up to date and it gave me the error again:

Windows update could not be installed because of error 2147942487 "The parameter is incorrect." (Command line: ""C:\windows\system32\wusa.exe"

1603 is non-existent. Let's make sure and look at the output of the command
winver

I assume the author meant 1607.  Drew - did you update drivers as suggested?

Yes, 1607. Yes I updated the chipset, graphics and BIOS drivers.

Run a Windows 10 Repair Install using the USB key and running Setup.

Go to the Media Creation Link:
https://www.microsoft.com/en-us/software-download/windows10

For Windows 10 V1709 and prior:
Windows 10 is running, so click on the Download button (not Upgrade Button, select Open (Run) but NOT Save. Allow the program to run. Allow drivers to update. Then select Keep Everything.
You can also Keep Data, or Keep Nothing. Keep Nothing is very close to a clean install.

For Windows 10 V1803:

If the Repair will not run as per above:
Create a USB Windows Installation key and then run Setup on the USB Key.
This will launch the Repair and proceed normally.

Self-solved by the author. Author forgot to close.