I am getting the same error as Julius (see link and error below). But I'm a newbie and don't understand how to use sudo as @cfr suggests in the comments.
Trouble installing Letter Gothic font
Jeremys-MacBook-Pro:documents jeremy$ texlua install-getnonfreefonts
Detected System: x86_64-darwin
Detected Installation: /usr/local/texlive/2017
mkdir /usr/local/texlive/2017/texmf-dist/scripts/getnonfreefonts ... [failed]
Sorry, couldn't create directory /usr/local/texlive/2017/texmf-dist/scripts/getnonfreefonts
sudobefore the command you wish to execute. you will be prompted for your admin password and than the command will be executed. I.e.:sudo texlua install-getnonfreefonts– naphaneal Mar 22 '18 at 17:01