GitHub Actions: Invalid value workflow reference: references to workflows must be rooted in '.github/workflows'

Hi!
I’m having trouble setting up a GitHub Workflow that would allow for automated application testing.

I’ve created .github/workflows directory in the root of my project, and inside of that directory I’ve created tests.yml file.

I need a Postgres DB for my application to start, and run the tests. I followed the article here

Repository code is contained in is private, and belongs to an organisation I am an Admin of.

Contents of my tests.yml look like this:

name: tests
on: [push, pull_request]

jobs:
  container-job:
    container: "swift:latest"
    runs-on: ubuntu-latest
    uses: actions/checkout@v2
    services:
      postgres:
        env:
          POSTGRES_DB: example-db
          POSTGRES_PASSWORD: example
          POSTGRES_USER: example
        image: postgres
        options: "--health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5"
    steps:
      - name: "Check out repository code"
        uses: actions/checkout@v2
      
      - name: "Run Swift Tests"
        run: "swift test --enable-test-discovery --sanitize=thread"
        env:
          DATABASE_HOST: postgres
          DATABASE_PASSWORD: example
          DATABASE_PORT: 5432
          DATABASE_USERNAME: example

Upon commit, GitHub tries to run the workflow, but fails with this message:

Error : .github#L1
invalid value workflow reference: references to workflows must be rooted in '.github/workflows'

I also receive an email that says

.github/workflows/tests.yml: No jobs were run

Could anyone please advise what an issue is? Is it something to do with repository being private & GitHub Actions Workflow not having sufficient permissions? I’m really confused as the .github/workflows/tests.yml seems to be exactly where it needs to be.

Thank you for your help in advance!