> в Модели Комментарии нет связи author  там связь называется user (можно посмотреть в scheme.rb)
Всё так
  create_table "comments", force: :cascade do |t|
    t.text "body", default: "", null: false
    t.bigint "user_id", null: false
< ... >
    t.index ["user_id"], name: "index_comments_on_user_id"
  end
Тем не менее не понятно
Парамсы берутся из factories.
Далее запускаем тест
      it "create a new comment" do
        post comments_path, params: { commentable_type: comment.class, commentable_id: comment.id }
        expect(Comment.first.body).to eq(comment)
      end
и он падает на
  1) Comments POST /comments/ user is sign in create a new comment
     Failure/Error: params.require(:comment).permit(:body, :commentable_type, :commentable_id)

     ActionController::ParameterMissing:
       param is missing or the value is empty: comment
       Did you mean?  commentable_id
                      commentable_type
                      controller
                      action
     # ./app/controllers/comments_controller.rb:36:in `comment_params'
     # ./app/controllers/comments_controller.rb:32:in `load_commentable!'
     # ./spec/requests/comments_spec.rb:21:in `block (4 levels) in <top (required)>'
При дебаге вижу только юзера
pry(#<RSpec::ExampleGroups::Comments::POSTComments::UserIsSignIn>)> User.all
=> [#<User id: 1557, email: "maynard.batz@bauch-rowe.name", first_name: "Esteban", last_name: "Dach", created_at: "2022-07-19 18:36:44.366152000 +0000", updated_at: "2022-07-19 18:36:44.366152000 +0000", username: "@lala", online: false>]
[2] pry(#<RSpec::ExampleGroups::Comments::POSTComments::UserIsSignIn>)> Task.all
=> []
[3] pry(#<RSpec::ExampleGroups::Comments::POSTComments::UserIsSignIn>)> Project.all
=> []