AWS Cloud9でRuby on Railsの環境構築

前提・実現したいこと

AWS Cloud9でRuby on Railsの環境構築を進めているのですが、上手くいきません。

参考サイト:https://remonote.jp/aws-cloud9-ruby-on-rails

cloud9でサーバーを起動する為にrails sと入力、

ec2-user:~/environment/test_app $ rails s
=> Booting Puma
=> Rails 5.0.7.2 application starting in development on http://localhost:8080
=> Run rails server -h for more startup options
Puma starting in single mode...

Version 3.12.1 (ruby 2.6.0-p0), codename: Llamas in Pajamas
Min threads: 5, max threads: 5
Environment: development
Listening on tcp://localhost:8080

Use Ctrl-C to stop

このような表示が出た後、Preview Running Applicationを実行すると、ターミナルの右側のページに以下のようなエラーが出ます。

発生している問題・エラーメッセージ

Gem::LoadError

Specified 'sqlite3' for database adapter, but the gem is not loaded. Add gem 'sqlite3' to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).

Extracted source (around line #176)

　 require path_to_adapter
174 rescue Gem::LoadError => e
175 raise Gem::LoadError, "Specified '#{spec[:adapter]}' for database
adapter, but the gem is not loaded. Add gem '#{e.name}' to your Gemfile
(and ensure its
176 version is at the minimum required by ActiveRecord)."
177 rescue LoadError => e
178 raise LoadError, "Could not load '#{path_to_adapter}'. Make sure that the adapter in config/database.yml is valid. If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add the necessary adapter gem to the Gemfile.", e.backtrace
end