Railsチュートリアル11章 Heroku SendGridが動作しません(涙)

Rails チュートリアル 11章の最後、本番環境にデプロイし、新規ユーザー登録の画面で"create my account" ボタンを押すと、下記のサインが出てきます。deployする前のrails testは全てグリーン、おそらくコードも間違いはないと思うのですが、config/environments/production.rb の該当箇所だけ下記いたします。何とか乗り越えたいと願っております。不足しております情報がありましたらご教示ください。何卒ご指導のほどよろしくお願いいたします。

<Herokuのサイン>

We're sorry, but something went wrong.
If you are the application owner check the logs for more information.

<herokuのログ>

2020-06-24T12:10:45.771082+00:00 app[web.1]:

2020-06-24T12:10:45.771082+00:00 app[web.1]: ----==_mimepart_5ef342c5b907b_a2b1320468f1c70566--
2020-06-24T12:10:45.771083+00:00 app[web.1]:
2020-06-24T12:10:45.771633+00:00 app[web.1]: I, [2020-06-24T12:10:45.771557 #10] INFO -- : [5586b325-0aec-4c69-ba59-808108915ad3] Completed 500 Internal Server Error in 1340ms (ActiveRecord: 14.7ms | Allocations: 6005)
2020-06-24T12:10:45.773148+00:00 app[web.1]: F, [2020-06-24T12:10:45.772901 #10] FATAL -- : [5586b325-0aec-4c69-ba59-808108915ad3]
2020-06-24T12:10:45.773149+00:00 app[web.1]: [5586b325-0aec-4c69-ba59-808108915ad3] ArgumentError (SMTP-AUTH requested but missing secret phrase):
2020-06-24T12:10:45.773150+00:00 app[web.1]: [5586b325-0aec-4c69-ba59-808108915ad3]
2020-06-24T12:10:45.773150+00:00 app[web.1]: [5586b325-0aec-4c69-ba59-808108915ad3] app/models/user.rb:55:in send_activation_email' 2020-06-24T12:10:45.773151+00:00 app[web.1]: [5586b325-0aec-4c69-ba59-808108915ad3] app/controllers/users_controller.rb:28:in create'
2020-06-24T12:10:45.774894+00:00 heroku[router]: at=info method=POST path="/users" host=guarded-wildwood-41214.herokuapp.com request_id=5586b325-0aec-4c69-ba59-808108915ad3 fwd="58.159.14.129" dyno=web.1 connect=0ms service=1359ms status=500 bytes=1891 protocol=https

Ruby
1  #11.41 Railsのproduction環境でSendGridを使う設定
2  # Set this to true and configure the email server for immediate delivery to raise delivery errors.
3  config.action_mailer.raise_delivery_errors = true
4  config.action_mailer.delivery_method = :smtp
5  host = 'guarded-wildwood-41214.herokuapp.com'
6  config.action_mailer.default_url_options = { host: host }
7  ActionMailer::Base.smtp_settings = {
8    :address        => 'smtp.sendgrid.net',
9    :port           => '587',
10    :authentication => :plain,
11    :user_name      => ENV['SENDGRID_USERNAME'],
12    :user_password  => ENV['SENDGRID_PASSWORD'],
13    :domain         => 'heroku.com',
14    :enable_starttls_auto => true
15  }