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IMPORTANT UPDATE:
Ruby Sass is deprecated and will be unmaintained as of 26 March 2019.

* If you use Sass as a command-line tool, we recommend using Dart Sass, the new
  primary implementation: https://sass-lang.com/install

* If you use Sass as a plug-in for a Ruby web framework, we recommend using the
  sassc gem: https://github.com/sass/sassc-ruby#readme

* For more details, please refer to the Sass blog:
  http://sass.logdown.com/posts/7081811

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In the guide "Linuxbrew - libsass and sassc" we installed libsass, the sass library, and sassc, the libsass command line driver.
You need to uninstall those apps from Linuxbrew to be able to install the gems sass and Bootstrap-sass from Ruby, due to the libraries are replacements no compatibility.
References: 
Guide: Linuxbrew - libsass and sassc


