herokuデプロイでrake assets:precompileで失敗

Question

#実現したいこと
heroku+carrierwave+S3にてデプロイを行おうとし、以下のコマンドを実行しました.

git push heroku master

#エラーメッセージ、問題

Preparing app for Rails asset pipeline
remote:        Running: rake assets:precompile
remote:        rake aborted!
remote:        NoMethodError: undefined method `[]' for nil:NilClass

 !
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: a8360d84664bdeda2f248a42eac7233d7f58e783
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version a8360d84664bdeda2f248a42eac7233d7f58e783
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 game-reviewers.
remote: 
To https://git.heroku.com/app-name.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/app-name.git'

該当のソースコード

CarrierWave.configure do |config|
  case Rails.env
  when 'development', 'test'
    config.storage = :file
    config.cache_storage = :file
  else
    config.fog_credentials = {
      provider: 'AWS',
      aws_access_key_id: Rails.application.credentials.aws[:access_key_id],
      aws_secret_access_key: Rails.application.credentials.aws[:secret_access_key],
      region: ENV['AWS_REGION'],
    }
    config.fog_provider = 'fog/aws'
    config.storage :fog
    config.cache_storage = :fog
    config.fog_directory = ENV['AWS_S3_BUCKET']
    config.fog_public = false
  end
end

#試したこと
こちらの記事を参考にして環境変数の編集やheroku configでAWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_S3_REGION, AWS_S3_BUCKETの環境変数を定義してherokuダッシュボードのConfig Varsでも確認しました。

ターミナル上でも環境変数が定義されていることを確認しましたが、同様なエラーが出てしまいます。
どなたかご協力お願いします。

#補足情報
rubyは3.0.1 railsは6.0.4 bundlerに2.2.21を使用しています