deploy.yml

name: Deploy
on:
  workflow_run:
    workflows: [CI Build] # change this to match your build workflow name
    types: [completed]
    branches: [main]

jobs:
  deploy:
    runs-on: ubuntu-latest
    environment:
      name: your-environment-name-here
      url: https://your-app.example.com
    name: Deploy demo
    if: ${{ github.event.workflow_run.conclusion == 'success' }}
    steps:
      - uses: actions/checkout@v2
        with:
          fetch-depth: 0
      - name: Push to Heroku
        run: git push -f https://heroku:$HEROKU_API_KEY@git.heroku.com/$HEROKU_APP_NAME.git main
        env:
          HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}
          HEROKU_APP_NAME: your-heroku-app

I think it's the presence of environment

You are right.