When trying to serve your site using jekyll and you get an error like this:

jekyll serve
...
/Library/Ruby/Gems/2.6.0/gems/bundler-2.1.4/lib/bundler/spec_set.rb:86:in `block in materialize': Could not find public_suffix-4.0.6 in any of the sources (Bundler::GemNotFound)

don’t forget you need to use bundle exec jekyll serve