$sudo gem install sass Building native extensions. This could take a while... ERROR: Error installing sass: ERROR: Failed to build gem native extension. current directory: /Library/Ruby/Gems/2.3.0/gems/ffi-1.9.23/ext/ffi_c /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20180228-5487-11of4og.rb extconf.rb mkmf.rb can't find header files for ruby at /System/Library/