Just in case you happened to copy the
.rubocop.yml file from a different project like I did, in my case I just forgot to add the rubocop-rails gem, which includes the Rails/ cops.
Install it like any other gem and and require it in your
# Gemfile gem 'rubocop', require: false gem 'rubocop-rails', require: false
$ bundle install
# .rubocop.yml require: rubocop-rails