Can’t push to the heroku

Read this doc which will explain to you what to do.
https://devcenter.heroku.com/articles/buildpacks

Setting a buildpack on an application

You can change the buildpack used by an application by setting the buildpack value.
When the application is next pushed, the new buildpack will be used.

$ heroku buildpacks:set heroku/php

Buildpack set. Next release on random-app-1234 will use heroku/php.
Run git push heroku master to create a new release using this buildpack.

This is whay its not working for you since you did not set it up.

… When the application is next pushed, the new buildpack will be used.

You may also specify a buildpack during app creation:

$ heroku create myapp --buildpack heroku/python