pipeline failing, not able to push changes to heroku

[SHAMIM AHMAD]

I have been trying to push my changes to heroku app deployment platform. I am exactly what is wrong with it. I am getting this error message which pipeline fails :

+ bash ./deploy.bash origin 
Initializing push to the specified environment: origin 
Permission denied (publickey).
fatal: Could not read from remote repository.Please make sure you have the correct access rights
and the repository exists.

I have generated SSH key on my machine. Uploaded public key to heroku account and private key in my bitbucket account as an environment variable.

bitbucker-pipeline.yml

pipelines:
  branches:
    master:
      - step:
          script:
          - mkdir -p ~/.ssh
          - cat known_hosts >> ~/.ssh/known_hosts
          - (umask 077; echo $MY_SSH_KEY | base64 -di > ~/.ssh/my_ssh_key)
          - bash ./deploy.bash origin

deploy.bash

MESSAGE="Initializing push to the specified environment: $1"

if [ $1 == "origin" ]; then
    echo $MESSAGE
    git push $1 $BITBUCKET_BRANCH:master
else
    echo "The specified environment is not valid. Use \"origin\"."
fi