Compiler does not work on MacOS High Sierra

I have MacOS High Sierra. The built-in compiler does not work.
MacBook-Pro-Mac:landio mac$ scss --watch scss:css-compiled
-bash: /usr/local/bin/scss: /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby: bad interpreter: No such file or directory

Installing cocoapods and RVM did not help. As I understand it, installing Wellington can help, but what exactly to do is not clear.

Please, who uses Wellington, explain what to do next?

1. Install Homebrew
2. $ brew install wget
3. $ brew create https://foo.com/bar-1.0.tgz
4. $ brew install wellington
5. What does it mean: "To take advantage of it to compile and scss folder into a css-compiled folder as in the example above you can use this gist. https://gist.github.com/rhukster/bcfe030e419028422d5e7cdc9b8f75a8"?
   I changed original file wellington-master/scripts/watch.sh, am i right?
6. what's next?
   Should I use the built-in compiler? Or do I need to install Gulp for example?

I am returning to working with Grav after a couple years away from all design work.

In trying re-familiarising myself with theme customisation, I am struggling little with this. I have had a little bit of experience with SCSS in the past (again not for some time), though struggling to get Wellington up and running.

For example, I am unsure how to locate the original watch.sh file so as to edit it with the gist linked to (I am a bit fuzzy).

Anyone able to point to steps for achieving this?

thanks

@Serebrennikov:
What does it mean: “To take advantage of it to compile and scss folder into a css-compiled folder as in the example above you can use this gist. https://gist.github.com/rhukster/bcfe030e419028422d5e7cdc9b8f75a8 ”?
I changed original file wellington-master/scripts/watch.sh, am i right?

Edit: I am working on OSX with MAMP.

I don't use Wellington, and I don't use any of the Grav themes styles. I use Bootstrap themes. A dependency file is usually attached to the theme. In short, all that's left to do is:

1. Download and install Node.js.
2. Download and install Gulp.js.
3. Navigate to the root themes directory and run npm install to install local dependencies listed in package.json.
4. Specify the location of the compiled and source files in the Gulp file 'gulpfile.js'.
5. You can now change the files and start the compile process - run 'gulp'. Or run the 'gulp watch' to run the compiler every time you change scss files.