I keep getting this error. I am using Mac. I generated a key and added it to heroku using
heroku config:add key=\"ssh-rsa blahblah=me@me.com\"
Copy the files in your .ssh path ex:
/c/Users//.ssh/
to the git .ssh folder.
/c/Program Files/Git/.ssh/
If the .ssh folder doesn't exist in the Git directory simply create it then paste your files.
For some reason windows require that a .ssh path be in the user directory as well as the Git directory. ON default this is not added so you simply have to create it yourself.
After adding the files to Git's directory simply run the following command again:
git push heroku master