Question about Contract Testing validation

Hi guys,

I’m new to stoplight and trying to play a little bit with contract testing.

I’ve a scenario working (project ERA API - passes scenario - v3.0) but contract validation fails because of a non required response field not returned in the response:
Object has no key ‘ResourceError’

Could you list me what are the rules that will make the validation fails?

In addition, is it possible not to fail when an optional field is not present in the response?

Thanks a lot,