rubocop how do you fix Missing magic comment

Try running Rubocop with the -D option:

rubocop -D
Inspecting 1 file
C

Offenses:

spec/rails_helper.rb:1:1: C: Style/FrozenStringLiteralComment: Missing magic comment # frozen_string_literal: true.
require 'spec_helper'
^

Adding -D will cause Rubocop to print the name of the cop that was violated, in this case Style/FrozenStringLiteralComment. You can then search for that cop in the Rubocop documentation:

http://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/FrozenStringLiteralComment

This cop is designed to help upgrade to Ruby 3.0. It will add the comment # frozen_string_literal: true to the top of files to enable frozen string literals. Frozen string literals will be default in Ruby 3.0. The comment will be added below a shebang and encoding comment. The frozen string literal comment is only valid in Ruby 2.3+.

If you want to ignore it, add to your .rubocop.yml

Style/FrozenStringLiteralComment:
  Enabled: false

But may be you want to know what "Magic comment" is, especially if you're using Ruby 2.x

Just add

# frozen_string_literal: true

to the first line of each Ruby file. Or run

rubocop -a

to allow Rubocop to fix all offenses automatically that it is able to fix.

Btw. I like Rubocop and use it myself, but I wouldn't call the things it finds defects. I see the list more like suggestions or reasons for a discussion with my co-workers.

If you want to be more specific and run rubocop for only # frozen_string_literal: true you can use the --only flag option:

Run only the specified cop(s) and/or cops in the specified departments.

To view those files:

rubocop --only Style/FrozenStringLiteralComment

To autocorrect those specific files use the -A flag (as mentioned in a previous answer):

rubocop --only Style/FrozenStringLiteralComment -A

You can view more information about autocorrect here.