rails testを正しく動作させたい

Question

前提・実現したいこと

rails testを正しく動作させたい

発生している問題・エラーメッセージ

プログラミング初心者でRailsチュートリアル３章、「最初のテスト」学習中に次のようなメッセージがでてきてしまいました。

Error:
StaticPagesControllerTest#test_should_get_help:
ActionView::Template::Error: Webpacker can't find application in /home/ubuntu/environment/sample_app/public/packs-test/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
   unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}

    app/views/layouts/application.html.erb:9
    test/controllers/static_pages_controller_test.rb:10:in `block in <class:StaticPagesControllerTest>'


rails test test/controllers/static_pages_controller_test.rb:9

E

Error:
StaticPagesControllerTest#test_should_get_home:
ActionView::Template::Error: Webpacker can't find application in /home/ubuntu/environment/sample_app/public/packs-test/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
   unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}

    app/views/layouts/application.html.erb:9
    test/controllers/static_pages_controller_test.rb:5:in `block in <class:StaticPagesControllerTest>'


rails test test/controllers/static_pages_controller_test.rb:4



Finished in 2.118673s, 0.9440 runs/s, 0.0000 assertions/s.
2 runs, 0 assertions, 0 failures, 2 errors, 0 skips

チュートリアル通りであればまだコードに変更を加える前なので
すべてのテストが成功するはずです

試したこと

https://note.com/hbkk/n/n032e39e19e31
上記のサイトを参考にし

$bin/yarn
$bin/webpack

のコマンドを打つと

warning ../../package.json: No license field                          
yarn run v1.22.15
warning ../../package.json: No license field
error Command "webpack" not found.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

このようなエラーがでてきてしまいました。

補足情報（FW/ツールのバージョンなど）

Cloud9
Rails 6.0.3
Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

gem 'rails',      '6.0.3'
gem 'puma',       '4.3.6'
gem 'sass-rails', '5.1.0'
gem 'webpacker',  '4.0.7'
gem 'turbolinks', '5.2.0'
gem 'jbuilder',   '2.9.1'
gem 'bootsnap',   '1.4.5', require: false

group :development, :test do
  gem 'sqlite3', '1.4.1'
  gem 'byebug',  '11.0.1', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  gem 'web-console',           '4.0.1'
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.1.0'
  gem 'spring-watcher-listen', '2.0.1'
end

group :test do
  gem 'capybara',                 '3.28.0'
  gem 'selenium-webdriver',       '3.142.4'
  gem 'webdrivers',               '4.1.2'
  gem 'rails-controller-testing', '1.0.4'
  gem 'minitest',                 '5.11.3'
  gem 'minitest-reporters',       '1.3.8'
  gem 'guard',                    '2.16.2'
  gem 'guard-minitest',           '2.4.6'
end
  
group :production do
  gem 'pg', '1.1.4'
end