Github actions on pull request and master branch

I think you are just missing a colon after pull_request. This works for me.

on:
  pull_request:
  push:
    branches: master
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - name: Test
        run: echo "done"

Explanation

Each trigger has to be defined as a property that defines an object.

Each object defines overrides for default settings.

There are 3 possible syntax you can use:

Minimal syntax:

on:
  pull_request:
  push: { branches: [master] }

Explicit syntax:

on:
  pull_request: {}
  push: { branches: [master] }

Extendable syntax:

on:
  pull_request:
  push: 
    branches: 
      - master

When using a version control system the latter may be most useful as diff viewers can always easily distinguish* between different lines.

*Although modern diff viewers can also easily distinguish inline differences.