Why can't I deploy my project to heroku?

Question

Django

Hello everyone!)
For the first time I encountered a deployment problem on heroku, the description of which is:

! No default language could be detected for this app.
HINT: This occurs when Heroku cannot detect the buildpack to use for this application automatically.
See https://devcenter.heroku.com/articles/buildpacks
! Push failed

The project has the following structure:

Procfile is present and has the following content:
web: gunicorn core.wsgi --log-file -
requirements.txt:

Django==2.1.4
gunicorn==19.9.0
pytz==2018.7

Everything is in the root of the project, but it keeps saying that it cannot determine the buildpack.
What could be wrong?