The problem was that,
@actions/glob library did not return any files even with a global matching pattern,
const glob = require("@actions/glob") const globber = await glob.create("**") const files = await globber.glob() // files = 
To access the file in GitHub action, one needs to check out a repository to access files for.
name: Report broken URLs on: push jobs: report_job: runs-on: ubuntu-latest steps: # 👇 Check out the repository # to be able to access the repository files # in "my_action" - name: Checkout repository uses: actions/checkout@v2 - name: Validate repository URLs and report broken link(s) uses: my_action@master