Install Jekyll on macOS

https://jekyllrb.com/docs/installation/macos/

Install Ruby instead of using system Ruby.

brew install chruby ruby-install
ruby-install ruby

Create ~/.oh-my-zsh/custom/ruby.zsh to auto switch ruby.

source $(brew --prefix)/opt/chruby/share/chruby/chruby.sh
source $(brew --prefix)/opt/chruby/share/chruby/auto.sh
chruby ruby-x.x.x

Then run:

gem install jekyll bundle
cd /destination/of/jekyll/site
bundle install