HEROKU : deploy a symfony 4 app push failed - heroku

Question

josh#josh:~/my_projects/la_manne$ git push heroku master
Énumération des objets: 3, fait.
Décompte des objets: 100% (3/3), fait.
Écriture des objets: 100% (3/3), 230 octets | 230.00 Kio/s, fait.
Total 3 (delta 0), réutilisés 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Building on the Heroku-20 stack
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: !
remote: ! ## Warning - The same version of this code has already been built: 73ed9a909f2a7e80a0a45f0f5d86aa6c7fdd70c1
remote: !
remote: ! We have detected that you have triggered a build from source code with version 73ed9a909f2a7e80a0a45f0f5d86aa6c7fdd70c1
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 lamanne.
remote:
To https://git.heroku.com/lamanne.git
! [remote rejected] master -> master (pre-receive hook declined)
error: impossible de pousser des références vers 'https://git.heroku.com/lamanne.git'

Answer 1

remote: -----> Building on the Heroku-20 stack
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
https://github.com/heroku/heroku-buildpack-php#usage
You'll need to use at least an empty composer.json in your
application.
$ echo '{}' > composer.json
$ git add composer.json
$ git commit -m "add composer.json for PHP app detection"
If you also have files from other frameworks or languages that could
trigger another buildpack to detect your application as one of its
own, e.g. a package.json which might cause your code to be detected
as a Node.js application even if it is a PHP application, then you
need to manually set your application to use this buildpack:
$ heroku buildpacks:set heroku/php
This will use the officially published version. To use the default
branch from GitHub instead:
$ heroku buildpacks:set https://github.com/heroku/heroku-buildpack-php
Please refer to Dev Center for further usage instructions.