Mysql2::Error::ConnectionError: Access denied for user 'root'@'localhost' (using password: NO)を解決する方法

database.ymlファイルでパスワードが設定されているか確認する
(この手順は検証していないがおそらく必要
パスワードは違うものでもいいかもしれない)

config/database.yml

# MySQL. Versions 5.5.8 and up are supported.
#
# Install the MySQL driver
#   gem install mysql2
#
# Ensure the MySQL gem is defined in your Gemfile
#   gem "mysql2"
#
# And be sure to use new-style password hashing:
#   https://dev.mysql.com/doc/refman/5.7/en/password-hashing.html
#
default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: root
  password: パスワード
  socket: /tmp/mysql.sock

development:
  <<: *default
  database: アプリ名_development

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: アプリ名_test

# As with config/credentials.yml, you never want to store sensitive information,
# like your database password, in your source code. If your source code is
# ever seen by anyone, they now have access to your database.
#
# Instead, provide the password or a full connection URL as an environment
# variable when you boot the app. For example:
#
#   DATABASE_URL="mysql2://myuser:mypass@localhost/somedatabase"
#
# If the connection URL is provided in the special DATABASE_URL environment
# variable, Rails will automatically merge its configuration values on top of
# the values provided in this file. Alternatively, you can specify a connection
# URL environment variable explicitly:
#
#   production:
#     url: <%= ENV["MY_APP_DATABASE_URL"] %>
#
# Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full overview on how database connection configuration can be specified.
#
production:
  <<: *default
  database: アプリ名_production
  username: アプリ名
  password: パスワード

開発するアプリのディレクトリに移動して

ターミナル

rails db:create   

を実行すればOK

実行結果

ターミナル

$ rails db:create
Created database 'アプリ名_development'
Created database 'アプリ名_test'

実行後↓