I tried (foolhardily, it appears) installing macOS Mojave Version10.4 on my MacBookAir, but encountered major problems--that seemed to have been (finally) fixed after a reinstallation and reloading of disk contents. But then when I tried running a Mathematica program, previously run many times, I encountered a new set of errors I had never seen before, that is:
CCompilerDriver`CreateLibrary::cmperr: Compile error: xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
Compile::nogen: A library could not be generated from the compiled function.
Perhaps I should contact Wolfram Support, and ask for a new copy of Mathematica Version 18.104.22.168 to install.