NameError - uninitialized constant Sass::Engine:
<p>I am getting  NameError - uninitialized constant Sass::Engine: when I run my sinatra app with sass gem.
installed ruby version 2.3.1 with rbenv and also installed sinatra, sass gem.</p>

<pre><code>require 'sinatra'
require 'slim'
require 'sass'  # required sass
require 'sinatra/reloader' if development?

get '/styles.css' do
    scss :styles #does not generate styles.css, styles.scss file is in /views folder
end

get '/' do
    slim :home
end

get '/about' do
    @title = "All About This Website"
    slim :about
end

get '/contact' do
     slim :contact #, :layout =&gt; :special
end

not_found do
   slim :not_found
end

get '/fake_error' do
   status 500
   "There's nothing wrong, really :P"
end
</code></pre>

<p><strong>Full error:</strong></p>

<blockquote>
  <p>NameError - uninitialized constant Sass::Engine:
  /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tilt-2.0.5/lib/tilt/sass.rb:13:in
  <code>prepare'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tilt-2.0.5/lib/tilt/template.rb:92:in
  </code>initialize'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/sinatra-1.4.7/lib/sinatra/base.rb:862:in
  <code>new'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/sinatra-1.4.7/lib/sinatra/base.rb:862:in
  </code>block in compile_template'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tilt-2.0.5/lib/tilt.rb:104:in
  <code>block in fetch'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tilt-2.0.5/lib/tilt.rb:103:in
  </code>fetch'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/tilt-2.0.5/lib/tilt.rb:103:in
  <code>fetch'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/sinatra-1.4.7/lib/sinatra/base.rb:841:in
  </code>compile_template'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/sinatra-1.4.7/lib/sinatra/base.rb:822:in
  <code>render'
    /home/tasqyn/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/sinatra-1.4.7/lib/sinatra/base.rb:687:in
  </code>scss'    main.rb:7:in `block in '</p>
</blockquote>

<p>What I am doing wrong?</p>

<p>and here is styles.scss:</p>

<pre><code>$red: #903;
$black: #444;
$white: #fff;
$main-font: Helvetica, Arial, sans-serif;

body {
  font-family: $main-font;
}

h1 {
  color: $red;
  font: 32px/1 $main-font;
}

header h1 {
  font-size: 40px;
  line-height: 80px;
  background: transparent url(/images/logo.png) 0 0 no-repeat;
  padding-left: 84px;
}

@mixin tabs ($background: blue, $color: yellow) {
  ul {
    list-style: none;
    margin: 0;
    padding: 0;
    background: $background;
    overflow: hidden;
  }

  li {
    float: left;
  }

  a {
    text-decoration: none;
    display: block;
    padding: 8px;
    background: $background;
    color: $color;
    &amp;:hover {
      background: darken($background, 20%);
    }
  }
}

nav {
  @@include tabs($background: $black, $color: $white);
  font-weight: bold;
}

p {
  font: 13px/1.4 $main-font;
}
</code></pre>