Schema.org (the vocabulary) doesn’t require any properties.
Google’s Structured Data Testing Tool is not a Schema.org validator. The tool primarily tests if your use of Schema.org conforms to Google’s own requirements for showing one of their Rich Snippets (or similar features/products).
For more details http://stackoverflow.com/a/33070334. But still we can try to fulfill Google’s requirements.
I am not sure. But there are some options like below