Check that you have the homebrew version of Ruby:
$ which ruby
/usr/local/opt/ruby/bin/ruby
And that you have version 3 or later:
ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin18]
Install Jekyll and bundler gems:
bundle install
Run local Jekyll server with live reload:
bundle exec jekyll serve --livereload