It seems like you're trying to access manage.py from the root of the filesystem. Dokku has most likely placed your application's files somewhere else. Assuming that the path /var/lib/herokuish/buildpacks/buildpack-python/tests/python-django/requirements.txt is on the container, manage.py should also be in the same directory. So, you will need to go to that directory first:

cd /var/lib/herokuish/buildpacks/buildpack-python/tests/python-django/

and then run manage.py:

python manage.py db init