Heroku: Git error after fork application

Setting:

I have a Node.js app (MEAN fullstack) (i.e. myApp) deployed to heroku that works great . I am using standard git push

$ git push heroku master

      

        
        
        
      

    

I wanted to create an intermediate replica of the application and use the heroku fork for that (as per https://devcenter.heroku.com/articles/fork-app ).

$ heroku fork -a myApp myApp-staging

      

        
        
        
      

    

The middleware also works great with its own database and has its own set of config variables (expected). !!! IMPORTANT

I also created a new git remote

$ git remote add staging git@heroku.com:myApp-staging.git
// repo address taken from "heroku info -a myApp-staging"

      

        
        
        
      

    

Problem:

1) If I try to push to production i.e. "git push heroic master" works great.

2) If I try to click on a stage

$ git push staging master

      

        
        
        
      

    

I am getting "Permission denied (publickey)".
3) If I try to clone my git repo staging application to a different directory, I get an empty repo

$ heroku git:clone -a industryhub-staging
// warning: You appear to have cloned an empty repository

      

        
        
        
      

    

Where is my staging repo? What Heroku code is running on my staging instance? Finally - how do I click on a stage?