macOS comes with its own version of Ruby which will not allow you to install Gems. To start developing in Ruby, we first have to install our own version of Ruby.

It is generally recommended to use a version manager for Ruby, like rbenv to manage different versions of Ruby in various projects.

brew install rbenv

Add this line to your .zshrc. This will setup rbenv including adding rbenv to your $PATH

eval "$(rbenv init -)"

See list of stable Ruby versions to install

rbenv install -l

Install the version of Ruby that you want

rbenv install <version>

State which version of Ruby you want to use as the global version

rbenv global <version>