Mac Pro (2018) Boot/Update/Shutdown Issues (Blackscreen and "macOs coud not be installed on your computer"
Possibly a Combo Update could remedy the situation, as your Mac seems to not like the update, or it could be lying to itself about having which prerequisites are required, or more likely, the package appears to be either unsigned or un-executable.

The Combo Update for macOS 10.13.4 is available from Apple at This Link