Modern CSS with Tailwind: Source code not working

Hi

I am having issues running the example code, tried on two machines (one I did install Ruby 3.0.0, but the this one is on one with an older Ruby version):

❯ bin/setup
== Installing dependencies ==
Warning: the running version of Bundler (2.1.4) is older than the version that created the lockfile (2.2.3). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.2.3`.
Your Ruby version is 2.6.3, but your Gemfile specified 3.0.0
Warning: the running version of Bundler (2.1.4) is older than the version that created the lockfile (2.2.3). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.2.3`.
Your Ruby version is 2.6.3, but your Gemfile specified 3.0.0

I think the main issue is the Gemfile specifiying Ruby 3, the Bundler is easy to solve.
Any help appreciated.