After successfully cloning my repo from heroku and added another remote
1/ git clone git@heroku.com:[APP].git
2/ git remote add bitbucket ssh://git@bitbucket
Recently I faced the same issue. I got the following error:
repository access denied. access via a deployment key is read-only.
You can have two kinds of SSH keys:
I simply removed my repository SSH key and added a new SSH key to my account and it worked well.
I hope it helps someone. Cheers