Github Actions: No runner matching the specified labels was found: ubuntu-latest

My organisation uses "GitHub Enterprise Server 3.0.1" (in case this has any impact).

I created a .github/workflows/scala.yml file in my project. The Github Action gets triggered just fine but it dies with this error:

"No runner matching the specified labels was found: ubuntu-latest."

I tried to change it to ubuntu-20.04 and now I get:

"No runner matching the specified labels was found: ubuntu-20.04."

Same result, no matter if I trigger the action manually or check-in/push a change to my Development branch: the action triggers but fails with the above errors (and I get an email from GitHub telling me my build-job failed). I have a feeling this might be a YAML parsing issue. I confirm that there are no spaces nor weird/invisible characters after the "ubuntu-latest" string/line (I checked in my Hex editor). Any ideas?

Here is my scala.yml:

name: Scala CI

on:
  workflow_dispatch:
  push:
    branches:
    - Development
  pull_request:
    branches:
    - Development

jobs:
  build-job:
    runs-on: ubuntu-20.04

    steps:
    - uses: actions/checkout@v2
    - name: Set up JDK 1.8
      uses: actions/setup-java@v1
      with:
        java-version: 1.8
    - name: Run tests
      run: sbt test