【ruby on rails】herokuデプロイ

$ git push heroku masterすると以下のエラーが出ます。

NoMethodError: Cannot load database configuration:undefined method `[]' for nil:NilClass

ターミナル

$ RAILS_ENV=development bin/rails assets:precompileの結果

mart@shigeruuraguchimasatonoMacBook-Pro myblog02 % RAILS_ENV=development bin/rails assets:precompile 
yarn install v1.22.4
[1/4] ????  Resolving packages...
success Already up-to-date.
✨  Done in 0.05s.

$ RAILS_ENV=production bin/rails assets:precompileの結果

mart@shigeruuraguchimasatonoMacBook-Pro myblog02 % RAILS_ENV=production bin/rails assets:precompile 
yarn install v1.22.4
[1/4] ????  Resolving packages...
success Nothing to install.
success Saved lockfile.
✨  Done in 0.05s.
I, [2020-12-16T12:33:03.824189 #7543]  INFO -- : Writing /Users/mart/myblog02/public/assets/application-f11ff91e826976101c0618dd53de0c322bf29547aa6eb5ef361086beae31a1d1.js
I, [2020-12-16T12:33:03.824707 #7543]  INFO -- : Writing /Users/mart/myblog02/public/assets/application-f11ff91e826976101c0618dd53de0c322bf29547aa6eb5ef361086beae31a1d1.js.gz
I, [2020-12-16T12:33:13.127223 #7543]  INFO -- : Writing /Users/mart/myblog02/public/assets/application-89ff2b0b13c65904beddb9cb1c007267d2121137ac074de709685c9d5e9bfe13.css
I, [2020-12-16T12:33:13.127390 #7543]  INFO -- : Writing /Users/mart/myblog02/public/assets/application-89ff2b0b13c65904beddb9cb1c007267d2121137ac074de709685c9d5e9bfe13.css.gz

ruby 2.5.1
rails 5.2.3
postgresql

database.yml

# PostgreSQL. Versions 9.1 and up are supported.
#
# Install the pg driver:
#   gem install pg
# On OS X with Homebrew:
#   gem install pg -- --with-pg-config=/usr/local/bin/pg_config
# On OS X with MacPorts:
#   gem install pg -- --with-pg-config=/opt/local/lib/postgresql84/bin/pg_config
# On Windows:
#   gem install pg
#       Choose the win32 build.
#       Install PostgreSQL and put its /bin directory on your path.
#
# Configure Using Gemfile
# gem 'pg'
#
default: &default
  adapter: postgresql
  encoding: unicode
  username: <%= Rails.application.credentials.db[:username] %>
  password: <%= Rails.application.credentials.db[:password] %>
  # host: localhost
  # For details on connection pooling, see Rails configuration guide
  # http://guides.rubyonrails.org/configuring.html#database-pooling
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: myblog02_development

  # The specified database role being used to connect to postgres.
  # To create additional roles in postgres see `$ createuser --help`.
  # When left blank, postgres will use the default role. This is
  # the same name as the operating system user that initialized the database.
  #username: myblog02

  # The password associated with the postgres role (username).
  #password:

  # Connect on a TCP socket. Omitted by default since the client uses a
  # domain socket that doesn't need configuration. Windows does not have
  # domain sockets, so uncomment these lines.
  #host: localhost

  # The TCP port the server listens on. Defaults to 5432.
  # If your server runs on a different port number, change accordingly.
  #port: 5432

  # Schema search path. The server defaults to $user,public
  #schema_search_path: myapp,sharedapp,public

  # Minimum log levels, in increasing order:
  #   debug5, debug4, debug3, debug2, debug1,
  #   log, notice, warning, error, fatal, and panic
  # Defaults to warning.
  #min_messages: notice

# 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: myblog02_test

# As with config/secrets.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 as a unix environment variable when you boot
# the app. Read http://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full rundown on how to provide these environment variables in a
# production deployment.
#
# On Heroku and other platform providers, you may have a full connection URL
# available as an environment variable. For example:
#
#   DATABASE_URL="postgres://myuser:mypass@localhost/somedatabase"
#
# You can use this database configuration with:
#
#   production:
#     url: <%= ENV['DATABASE_URL'] %>
#
production:
  <<: *default
  database: myblog02_production
  username: <%= ENV['DB_USER'] %>
  password: <%= ENV['DB_PASSWORD'] %>

apprication.scss

/*
 * This is a manifest file that'll be compiled into application.css, which will include all the files
 * listed below.
 *
 * Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
 * vendor/assets/stylesheets directory can be referenced here using a relative path.
 *
 * You're free to add application-wide styles to this file and they'll appear at the bottom of the
 * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
 * files in this directory. Styles in this file should be added after the last require_* statement.
 * It is generally better to create a new file per style scope.
 *
 *= require_tree .
 *= require_self
 */

 @import "bulma";
 @import "font-awesome-sprockets";
 @import "font-awesome";

 .notification:not(:last-child){
   margin-bottom: 0;
 }

 .profile_image {
  border-radius: 50%;
 }

#bg-video{
  position: fixed;
  right: 0;
  bottom: 0;
  min-width: 100%;
  min-height: 100%;
  width: auto;
  height: auto;
  z-index: -100;
  background: url('P_20181107_220321.jpg') no-repeat;
  background-size: cover;
 }

apprication.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require jquery3
//= require_tree .

試したこと

1.database.ymlのproduction環境指定欄を削除
リンク内容

2.GitHubリポジトリ上で RAILS_MASTER_KEY を設定する
リンク内容

3.config>environments>production.rbの下記の通り編集

config.assets.js_compressor = :uglifier
#これをコメントアウトする

4.config/application.rbにconfig.assets.initialize_on_precompile = falseを追加

require_relative 'boot'

require 'rails/all'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module Myblog02
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 5.2

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration can go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded after loading
    # the framework and any gems in your application.
    config.time_zone = 'Tokyo'
    config.assets.initialize_on_precompile = false
  end
end

当方、プログラミングを始めて半年の初学者です。
いろいろ試してみましたがうまくいきません。
どなたかご教授頂けないでしょうか。よろしくお願いします。