I had the same issue, but later I found out that I forgot to create an app before deploying it. Try the following steps in terminal.
heroku login
heroku create
I am just learning heroku and often forget the steps so I wrote an article about it. You can find it here: https://medium.com/@saurav.panthee/deploy-flask-app-to-heroku-under-3-minutes-2ec1c0bc403a