Connect to Dropbox isn't working
This might be caused by the use of Heroku Fork. If the content of your git repository is empty, it means there wasnt a deploy to the app using the Git service. The Dropbox sync system requires a git repo to be present on the Heroku side in order to work or the app to have never been deployed.
You'll need a git repository containing your application code, and then you can connect it to Heroku by running
heroku git:remote -a your-app-name from your app directory and then perform a git deploy by running
git push heroku master.
An alternative is deleting the app and recreating it.
Once you've done this one time you shouldn't need to again.