From another thread: https://answers.microsoft.com/en-us/windows/forum/windows\_10-update/failed-install-2018-03-cumulative-update-for/140c7c68-8cd9-44fb-a9b7-bf3241846af5
Before installing KB4093112, KB4088776 has to be installed correctly first. Try installing
KB4088776 first by doing this:
a) Search cmd in the Windows search bar. Right click on Command Prompt and click Run as Administrator
b) On the open Command Prompt window, copy and paste the commands (all at once)
net stop wuauserv
net stop cryptSvc
net stop bits
net stop msiserver
ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
ren C:\Windows\System32\catroot2 catroot2.old
net start wuauserv
net start cryptSvc
net start bits
net start msiserver
c) Download KB4088776 from https://www.catalog.update.microsoft.com/Search.aspx?q=KB4088776, and install it.
d) Restart your computer.
e) Do parts a) and b) again
f) Download the Delta update for KB4093112 from https://www.catalog.update.microsoft.com/Search.aspx?q=KB4093112, and install it