If you have created a bootable USB from the MCT, can you boot up with that and navigate to the Install screen and click on Repair your computer/Troubleshoot and select Command Prompt.
If you haven't created a bootable USB then you can do after reading -
https://www.microsoft.com/en-gb/software-download/windows10Insert the USB before downloading the MCT and it will auto create the bootable USB without the need for any 3rd party programs.
This will give you the latest version of Windows.
At the prompt enter bcdedit |find "osdevice"
For clarity that is a Pipe symbol before find and is the uppercase of \
Using your partition letter instead of the x I have exampled, enter -
dism /image:x:\ /cleanup-image /restorehealth
sfc /scannow /offbootdir=X:\ /offwindir=X:\Windows
Note the space before each /
Enter exit to close the cmd window, remove the install USB and reboot and then either use the Settings cog/Update & Security to check for updates or try the May 2020 download again.
Let me know if those cmds report anything other than completed successfully or no integrity violations.
If you still have problems then insert the install USB - open Windows/File Explorer then double click on the USB drive which will open to its files where you would then double click on setup Application.
This will perform a repair install which doesn't affect personal files or installed programs.