No clear examples for rails project

I'm new to Ruby on Rails, Github Actions and Coveralls, and I can't seem to get this github action working. It doesn't seem to be able to find the coverage report file after I run rspec tests.

My .coveralls.yml file looks like this:

on: [push, "pull_request"]

jobs:
  build:

    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v1
    - name: Set up Ruby 2.6
      uses: actions/setup-ruby@v1
      with:
        ruby-version: 2.6.x

    - name: Run rspec
      run: |
        gem install bundler
        bundle install
        bundle exec rspec spec

    - name: Coveralls
      uses: coverallsapp/github-action@master
      with:
        github-token: ${{ secrets.GITHUB_TOKEN }}

What am I missing?

Start with the pasted .coveralls.yml and the GitHub Actions workflow, then inspect how bundle exec rspec spec produces the coverage report before the Coveralls action runs. Done means the workflow consistently finds and uploads the generated report; the issue provides no named test or file beyond these configuration and command references.