brew install ruby

export PATH="/usr/local/opt/ruby/bin:$PATH"

which ruby
/usr/local/opt/ruby/bin/ruby

PATH="`ruby -e 'puts Gem.user_dir'`/bin:$PATH"

brew install libffi

export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"

gem install --user-install bundler jekyll     
Successfully installed bundler-2.2.15
Successfully installed jekyll-4.2.0
2 gems installed

jekyll
A subcommand is required. 
jekyll 4.2.0 -- Jekyll is a blog-aware, static site generator in Ruby

Usage:

  jekyll <subcommand> [options]

Options:
        -s, --source [DIR]  Source directory (defaults to ./)
        -d, --destination [DIR]  Destination directory (defaults to ./_site)
            --safe         Safe mode (defaults to false)
        -p, --plugins PLUGINS_DIR1[,PLUGINS_DIR2[,...]]  Plugins directory (defaults to ./_plugins)
            --layouts DIR  Layouts directory (defaults to ./_layouts)
            --profile      Generate a Liquid rendering profile
        -h, --help         Show this message
        -v, --version      Print the name and version
        -t, --trace        Show the full backtrace when an error occurs

Subcommands:
  compose               
  docs                  
  import                
  build, b              Build your site
  clean                 Clean the site (removes site output and metadata file) without building.
  doctor, hyde          Search site and print specific deprecation warnings
  help                  Show the help message, optionally for a given subcommand.
  new                   Creates a new Jekyll site scaffold in PATH
  new-theme             Creates a new Jekyll theme scaffold
  serve, server, s      Serve your site locally