rails I am a beginner.

I'm having trouble with Ruby on Rails Tutorial Chapter 11 11.4 "Mail Sending in Production".
The email from SendGrid will be delivered.

When connecting to the link from Activate in the email,

Cannot access this site .. The server IP address of .herokuapp.com was not found.
DNS_PROBE_FINISHED_NXDOMAIN

It is displayed like

.

• Try it

The following article was used as a reference.
https://www.tutorialfor.com/go.php?id=112723

(host = '.herokuapp.com' has changed to your environment)

production.rb

config.action_mailer.raise_delivery_errors = true
  config.action_mailer.delivery_method =: smtp
  host = '<your heroku app>.herokuapp.com'
  config.action_mailer.default_url_options = {host: host}
  ActionMailer :: Base.smtp_settings = {
    : address =>'smtp.sendgrid.net',
    : port =>'587',
    : authentication =>: plain,
    : user_name =>ENV ['SENDGRID_USERNAME'],
    : password =>ENV ['SENDGRID_PASSWORD'],
    : domain =>'heroku.com',
    : enable_starttls_auto =>true
  }

SENDGRID_USERNAME, SENDGRID_PASSWORD part
$heroku config: get SENDGRID_USERNAME
$heroku config: get SENDGRID_PASSWORD
I applied the user_name password that was displayed in, but I fixed it because another error occurred.
* We referenced https://www.tutorialfor.com/go.php?id=74492 .

account_activation.text

Hi<% = @ user.name%>,
Welcome to the Sample App! Click on the link below to activate your account:
<% = edit_account_activation_url (@ user.activation_token, email: @ user.email)%>

In the result of heroku logs,

Process exited with status 127
...
 Stopping all processes with SIGTERM
...
Process exited with status 143

There are three parts displayed in red with

. Is this related?
I'm in trouble without knowing what the error is.