やろうとしたこと

rails6で制作したアプリをデプロイしようとしたところエラーが発生し、行き詰まりました。

やったこと

HerokuCLIをインストール

 %brew tap heroku/brew && brew install heroku
中略
==> Upgrading heroku/brew/heroku
  7.54.1 -> 7.59.0

Herokuのバージョン確認

 %heroku --version
heroku/7.59.0 darwin-x64 node-v12.21.0

ログイン

% heroku login --interactive
Logged in as **************@********.co.jp

heroku上にアプリ名のディレクトリを作成

%heroku create icokkatest
Name icokkatest is already taken

正しくできたことの確認

%git config --list | grep heroku
remote.heroku.url=https://git.heroku.com/icokkatest.git
remote.heroku.fetch=+refs/heads/*:refs/remotes/heroku/*

ClearDBアドオンの追加

%heroku addons:add cleardb

Creating cleardb on ⬢ icokkatest... free
Created cleardb-trapezoidal-53860 as CLEARDB_GREEN_URL
Use heroku addons:docs cleardb to view documentation

設定変更

% heroku_cleardb=`heroku config:get CLEARDB_DATABASE_URL`
matsudatomohiro@Toms-MacBook-Pro icokkatest % heroku config:set DATABASE_URL=mysql2${heroku_cleardb:5}
Setting DATABASE_URL and restarting ⬢ icokkatest... done, v11
DATABASE_URL: mysql2://****************@us-cdbr-east-04.cleardb.com/heroku_******************?reconnect=true

暗号化の中身確認

% EDITOR="vi" bin/rails credentials:edit
File encrypted and saved.

コミット、プッシュしてmasterブランチへ反映させる。
heroku上で環境変数を設定

% heroku config:set RAILS_MASTER_KEY=`cat config/master.key` 
Setting RAILS_MASTER_KEY and restarting ⬢ icokkatest... done, v11
RAILS_MASTER_KEY: ***********************

環境変数を確認

% heroku config
=== icokkatest Config Vars
CLEARDB_DATABASE_URL: mysql://*************@us-cdbr-east-04.cleardb.com/heroku_**************?reconnect=true
CLEARDB_GREEN_URL:    mysql://***************@us-cdbr-east-04.cleardb.com/heroku_*******************?reconnect=true
DATABASE_URL:         mysql2://*****************@us-cdbr-east-04.cleardb.com/heroku_*******************?reconnect=true
RAILS_MASTER_KEY:     ********************************

Stackを指定

% heroku stack:set heroku-18 -a icokkatest
Setting stack to heroku-18... done

heroku上へアプリケーションをプッシュ

% git push heroku master

問題点

以下のようにビルドに失敗します。

remote:  !     Precompiling assets failed.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed

ログを読んでいくと以下の文言があったので、こちらが原因ではないかと考えて調べました。

NoMethodError: undefined method `[]' for nil:NilClass

herokuのログを調べたところ、こちらのメッセージが出てました。

https://dashboard.heroku.com/apps/f496a726-dbe1-418f-b082-e162bc26eac0/activity/builds/1d08535c-fd66-4580-8ba7-6ce7fe04386b

私にはこちらの情報からは特にどういったことをすれば良いのか解決策が見つからず困っております。
大変お手数ですが、どなたかご協力いただけるとありがたいです。

ターミナル上の結果（全文）

Enumerating objects: 466, done.
Counting objects: 100% (466/466), done.
Delta compression using up to 8 threads
Compressing objects: 100% (434/434), done.
Writing objects: 100% (466/466), 759.33 KiB | 13.32 MiB/s, done.
Total 466 (delta 178), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:

remote: 
remote: -----> Building on the Heroku-18 stack
remote: -----> Determining which buildpack to use for this app
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote: 			Detected buildpacks: Ruby,Node.js
remote: 			See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
remote: -----> Ruby app detected
remote: -----> Installing bundler 2.2.21
remote: -----> Removing BUNDLED WITH version in the Gemfile.lock
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.6.5
remote: -----> Installing dependencies using bundler 2.2.21
remote:        Running: BUNDLE_WITHOUT='development:test' BUNDLE_PATH=vendor/bundle BUNDLE_BIN=vendor/bundle/bin BUNDLE_DEPLOYMENT=1 bundle install -j4
remote:        Fetching gem metadata from https://rubygems.org/
remote:        Fetching gem metadata from https://rubygems.org/............
中略
remote:        Installing rails 6.0.4.1
remote:        Fetching webpacker 4.3.0
remote:        Fetching sass-rails 5.1.0
remote:        Fetching devise 4.8.0
remote:        Installing sass-rails 5.1.0
remote:        Installing devise 4.8.0
remote:        Installing webpacker 4.3.0
remote:        Bundle complete! 26 Gemfile dependencies, 76 gems now installed.
remote:        Gems in the groups 'development' and 'test' were not installed.
remote:        Bundled gems are installed into `./vendor/bundle`
remote:        Post-install message from sass:
remote:        
remote:        Ruby Sass has reached end-of-life and should no longer be used.
remote:        
remote:        * If you use Sass as a command-line tool, we recommend using Dart Sass, the new
remote:          primary implementation: https://sass-lang.com/install
remote:        
remote:        * If you use Sass as a plug-in for a Ruby web framework, we recommend using the
remote:          sassc gem: https://github.com/sass/sassc-ruby#readme
remote:        
remote:        * For more details, please refer to the Sass blog:
remote:          https://sass-lang.com/blog/posts/7828841
remote:        
remote:        Bundle completed (31.12s)
remote:        Cleaning up the bundler cache.
remote: -----> Installing node-v12.16.2-linux-x64
remote: -----> Installing yarn-v1.22.4
remote: -----> Detecting rake tasks
remote: -----> Preparing app for Rails asset pipeline
remote:        Running: rake assets:precompile
remote:        yarn install v1.22.4
remote:        [1/4] Resolving packages...
remote:        [2/4] Fetching packages...
remote:        info fsevents@2.3.2: The platform "linux" is incompatible with this module.
remote:        info "fsevents@2.3.2" is an optional dependency and failed compatibility check. Excluding it from installation.
remote:        info fsevents@1.2.13: The platform "linux" is incompatible with this module.
remote:        info "fsevents@1.2.13" is an optional dependency and failed compatibility check. Excluding it from installation.
remote:        [3/4] Linking dependencies...
remote:        warning " > webpack-dev-server@4.2.0" has unmet peer dependency "webpack@^4.37.0 || ^5.0.0".
remote:        warning "webpack-dev-server > webpack-dev-middleware@5.1.0" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
remote:        [4/4] Building fresh packages...
remote:        Done in 27.16s.
remote:        yarn install v1.22.4
remote:        [1/4] Resolving packages...
remote:        [2/4] Fetching packages...
remote:        info fsevents@2.3.2: The platform "linux" is incompatible with this module.
remote:        info "fsevents@2.3.2" is an optional dependency and failed compatibility check. Excluding it from installation.
remote:        info fsevents@1.2.13: The platform "linux" is incompatible with this module.
remote:        info "fsevents@1.2.13" is an optional dependency and failed compatibility check. Excluding it from installation.
remote:        [3/4] Linking dependencies...
remote:        warning " > webpack-dev-server@4.2.0" has unmet peer dependency "webpack@^4.37.0 || ^5.0.0".
remote:        warning "webpack-dev-server > webpack-dev-middleware@5.1.0" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
remote:        [4/4] Building fresh packages...
remote:        Done in 5.51s.
中略
remote:        /tmp/build_e1a02662/vendor/bundle/ruby/2.6.0/gems/sprockets-rails-3.2.2/lib/sprockets/rails/task.rb:66:in `block (2 levels) in define'
remote:        Tasks: TOP => assets:precompile
remote:        (See full trace by running task with --trace)
remote: 
remote:  !
remote:  !     Precompiling assets failed.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote:  !
remote:  ! ## Warning - The same version of this code has already been built: 4c12e116d989c469fd267deaef07e2f505e902f2
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version 4c12e116d989c469fd267deaef07e2f505e902f2
remote:  ! at least twice. One common cause of this behavior is attempting to deploy code from a different branch.
remote:  !
remote:  ! If you are developing on a branch and deploying via git you must run:
remote:  !
remote:  !     git push heroku <branchname>:main
remote:  !
remote:  ! This article goes into details on the behavior:
remote:  !   https://devcenter.heroku.com/articles/duplicate-build-version
remote: 
remote: Verifying deploy...
remote: 
remote: !	Push rejected to icokkatest.
remote: 
To https://git.heroku.com/icokkatest.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/icokkatest.git'