RailsアプリをHerokuにデプロイできない（No default language could be detected for this app.）

前提・実現したいこと

HerokuでRailsのテストアプリをデプロイする際に、エラーが表示されたので、いくつか試してみたのですが、何をすべきなのかご教示いただきたいです。

発生している問題・エラーメッセージ

user$ git push heroku master
Enumerating objects: 666, done.
Counting objects: 100% (666/666), done.
Delta compression using up to 8 threads
Compressing objects: 100% (627/627), done.
Writing objects: 100% (666/666), 234.78 KiB | 4.60 MiB/s, done.
Total 666 (delta 333), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote:  !     No default language could be detected for this app.
remote:                         HINT: This occurs when Heroku cannot detect the buildpack to use for this application automatically.
remote:                         See https://devcenter.heroku.com/articles/buildpacks
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to test-ttmanager.
remote: 
To https://git.heroku.com/test-ttmanager.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/test-ttmanager.git'
user$ heroku create --buildpack heroku/ruby
Creating app... done, ⬢ dry-hollows-61662
Setting buildpack to heroku/ruby... done
https://dry-hollows-61662.herokuapp.com/ | https://git.heroku.com/dry-hollows-61662.git
USERnoMacBook-puro:local_repository user$ git push heroku master
Enumerating objects: 666, done.
Counting objects: 100% (666/666), done.
Delta compression using up to 8 threads
Compressing objects: 100% (627/627), done.
Writing objects: 100% (666/666), 234.83 KiB | 4.52 MiB/s, done.
Total 666 (delta 333), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote:  !     No default language could be detected for this app.
remote:                         HINT: This occurs when Heroku cannot detect the buildpack to use for this application automatically.
remote:                         See https://devcenter.heroku.com/articles/buildpacks
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to test-ttmanager.
remote: 
To https://git.heroku.com/test-ttmanager.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/test-ttmanager.git'

試したこと

No default language could be detected for this app.
とあったので、調べた所、Heroku Buildpack for Rubyが必要だと考え、
$ heroku create --buildpack heroku/rubyを実行しました。

インストールができ、もう一度、git push heroku masterしたのですが、全く同じエラーが返ってきており、正直お手上げ状態です。こちらについて、解決策や、参考URLをご教示頂けますと幸いです。

補足情報（FW/ツールのバージョンなど）

• Rails：5．2．4
• DB:PostgreSQLを採用