To evaluate expressions in with configuration, you need to put your expression inside {{ }} .

Please try this:

name: CI
on:
  push:
    branches:
      - master
  schedule:

    - cron: '*/5 * * * *'
jobs:

  build:

    runs-on: ubuntu-latest

    steps:

    - uses: actions/checkout@v1

    - uses: actions/hello-world-docker-action@master

      with:

        who-to-greet: ${{ github.event_name != 'schedule' }}

You can refer to “About context and expressions” document:

https://help.github.com/en/actions/automating-your-workflow-with-github-actions/contexts-and-expression-syntax-for-github-actions#about-context-and-expressions