I know this has already been answered. But I would like to add my solution as it may helpful for others in the future..
A common key error is: Permission denied (publickey). You can fix this by using keys:add to notify Heroku of your new key.
In short follow these steps: https://devcenter.heroku.com/articles/keys
First you have to create a key if you don’t have one:
ssh-keygen -t rsa
Second you have to add the key to Heroku:
heroku keys:add