that's happened with me, because there is no /local file inside /usr
and also there is no /xcode inside /Library/Developer
and install.sh's code can't create new folder inside /usr and /Library/Developer.
so we should do that manually:
1- show hidden files
2- go to /usr, then create new folder with name "local"
3- go to /Library/Developer, then create new folder with name "xcode"
4- launch terminal
5- drag and drop "install.sh" to terminal, hit enter ...