You need to tell RuboCop to load the Rails extension. There are three ways to do this:

RuboCop configuration file

Put this into your .rubocop.yml.

require: rubocop-rails

Now you can run rubocop and it will automatically load the RuboCop Rails cops together with the standard cops.

Command line

$ rubocop --require rubocop-rails

Rake task do |task|
  task.requires << 'rubocop-rails'

Rails configuration tip

If you are using Rails 6.1 or newer, add the following config.generators.after_generate setting to your config/application.rb to apply RuboCop autocorretion to code generated by bin/rails g.

module YourCoolApp
  class Application < Rails::Application
    config.generators.after_generate do |files|
      parsable_files = files.filter { |file| file.end_with?('.rb') }
      unless parsable_files.empty?
        system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)

It uses rubocop -A to apply Style/FrozenStringLiteralComment and other unsafe autocorretion cops. rubocop -A is unsafe autocorrection, but code generated by default is simple and less likely to be incompatible with rubocop -A. If you have problems you can replace it with rubocop -a instead.