Estaba empujando el repositorio git actual en el heroku. Esa aplicación en línea fue desarrollada usando Scala e IntelliJ. Y no sé cómo solucionar este error.

$ git push heroku master
Counting objects: 3, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 531 bytes | 0 bytes/s, done.
Total 3 (delta 0), 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 salty-coast-14102.
remote: 
To https://git.heroku.com/salty-coast-14102.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/salty-coast-14102.git'