HerokuにpushしてからHeroku openした時に出てきたエラーです。

エラー

The `development` database is not configured for the `development` environment.

原因

database.ymlのインデントが間違えているから

解決

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: password
  host: db

development:
  <<: *default
  database: rails-front-demo-db

# 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: rails-front-demo-db
  host: <%= ENV.fetch("APP_DATABASE_HOST") { 'db' } %>

# 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: <%= ENV['APP_DATABASE'] %>
  username: <%= ENV['APP_DATABASE_USERNAME'] %>
  password: <%= ENV['APP_DATABASE_PASSWORD'] %>
  host: <%= ENV['APP_DATABASE_HOST'] %>

自動保存の設定のためいつの間にかインデントが変わっていることがあると学びました。
インデントを綺麗にしたらエラー解決しました。