"You don't have write permissions" error when attempting to install RuboCop

[xobs]

I was told by RuboCop (rubocop/rubocop#12130) that this is a bug in Rubygems.

I tried to follow the documentation on how to install RuboCop, but the instructions failed. The instructions on https://rubocop.org/ said to run:

$ gem install rubocop

However, this did not work on my setup:

$ gem install rubocop
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.6.0 directory.
$

I thought maybe there were additional setup instructions, such as maybe creating an environment, installing a different interpreter, or specifying an installation path, but they said that there weren't any additional instructions. They said that it was an issue with rubygems.

This is a default install on Mac using the system interpreter.