There is a fellow on the X-Plane forum who is having a heck of a time installing Hombrew.
The interesting point is that his home directory and other things are on an external SDD.
He has tired the command line install and a Uninstall (Install) master Unix executable; evidently with no success.
It looks like most of the errors that he is reporting are permission errors.
fatal: cannot copy ‘/Library/Developer/CommandLineTools/usr/share/git-core/templates/hooks/fsmonitor-watchman.sample’ to ‘/usr/local/Homebrew/.git/hooks/fsmonitor-watchman.sample’: Permission denied
So I am wondering if having his home on an external drive is the problem.