'error Command "webpack" not found.'

環境

rails :6.0.2
ruby :2.6.3p62

'error Command "webpack" not found.'のエラーが発生し、herokuへのデプロイが失敗します。

発生している問題・エラーメッセージ

$ git push heroku

remote:        Compiling...
remote:        Compilation failed:
remote:        yarn run v1.22.4
remote:        info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
remote:        
remote:        
remote:        error Command "webpack" not found.
remote:        
remote: 
remote:  !
remote:  !     Precompiling assets failed.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...

該当のソースコード

試したこと

$ yarn run

info Commands available from binary scripts: acorn, ansi-html, atob, autoprefixer, browserslist, css-blank-pseudo, css-has-pseudo, css-prefers-color-scheme, cssesc, errno, esparse, esvalidate, import-local-fixture, in-install, in-publish, js-yaml, jsesc, json5, loose-envify, miller-rabin, mime, mkdirp, multicast-dns, node-gyp, node-sass, nopt, not-in-install, not-in-publish, parser, regjsparser, rimraf, sassgraph, semver, sha.js, sshpk-conv, sshpk-sign, sshpk-verify, strip-indent, svgo, terser, uuid, webpack, webpack-cli, webpack-dev-server, which
error There are no scripts specified inside package.json.

(package.json内にスクリプトが指定されていません。)のエラー。

/package.json

{
  "name": "name",
  "private": true,
  "dependencies": {
    "@rails/webpacker": "4.2.2",
    "webpack": "^4.44.1",
    "webpack-cli": "^3.3.12"
  },
  "devDependencies": {
    "webpack-dev-server": "3.10.3"
  }
}

こちらに従いnode_modulesフォルダを削除してからwebpakerの再インストール
$ bundle exec rails webpacker:install

インストールは正常にされますが、状況変わらず。