Error on deploying with Git on Heroku

I’m not able to deploy in Heroku. I’m getting Permission Denied! everytime. I have my repository already exists.

$ git remote -v
heroku (fetch)
heroku (push)

But while I try to push in Heroku, I always get error.

$ git push -u heroku master
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

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