If you have a old ruby version (probably macOS built in), perhaps, you won't update gem or install cocoapods successfully.

May be like this:

YAML safe loading is not available. Please upgrade psych to a version that supports safe loading.

So, if you got problem like this

Here is the solution:

⚠️homebrew should be installed

brew install ruby
brew link --overwrite ruby
sudo gem update --system
sudo gem install cocoapods

You can copy each line and run from terminal.

BTW:

If you have a newer version of ruby, just run these to install cocoapods

sudo gem update --system
sudo gem install cocoapods