Heroku: Permission denied (publickey). fatal: Could not read from remote repository.

You have make sure, you have added the heroku rsa public key to the server from your local machine

Check whether you have a file called "id_rsa.pub" in the .ssh folder. If not you have generate the key. 

Follow the steps:

1. To generate the key : $ ssh-keygen -t rsa
(You will get these prompts, press enter and continue
Enter file in which to save the key (/Users/TestUser/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: )

2.  Add the key to Heroku server : $ heroku keys:add ~/.ssh/id_rsa.pub

Reference : https://devcenter.heroku.com/articles/keys