Hey there.
Are you following this tutorial?
I noticed for the other categories (Maven, Gradle, .NET) we explicitly ask to setup Java 11 with this step.
- name: Set up JDK 11
uses: actions/setup-java@v1
with:
java-version: 1.11
But we don’t have it listed for Other, even though it uses the same runs-on as Maven and Gradle (ubuntu-latest) 
Can you confirm this is the tutorial you’re following, and if so add this step after your Git checkout to see if that causes the issue to go away?