Why can't git resolve the hostname when I push to a valid SSH address?

Question

I am deploying an app on Heroku so I created a Heroku app from a repo and then did git push heroku master. When I do this it keeps giving me the error:

Answer 1

git@heroku.com:heroku-app.git is an SCP format for this ssh address.

It relies on a ~/.ssh/config file with a 'heroku.com' entry, which specify the user, the actual hostname, and if needed, the private/public key path.

host heroku.com
     user git
     hostname heroku.com
     identityfile ~/.ssh/yourPrivateKey

Again: heroku.com in 'heroku.com:heroku-app.git' is not an hostname: it is an entry in an ssh config file.
You could replace heroku.com by xxx: git push xxx:heroku-app.git, provided you have an xxx entry in the ~/.ssh/config file.