Referencing Assessments on Validation Rule for OpportunityObject

Trying to create a Groovy Script for a Validation Rule on the Opportunity Object. Essentially stating:

if(OpportunityType_c == 'NEWANNUITYACCOUNT' && Revenue >= 100000 && WinProb >= 75.00 && StatusCode == 'Open')

AND

Related Assessment doesn't exist or is not completed, there's an error message requesting Sales Rep to complete an Assessment.

Even though I see the Assessment fields on the "Fields" section of the Expression Palette, I can't reference any of the fields even when putting "AssessmentsVO?. [FIELD NAME]".

I understand there could be multiple Assessments templates but in our UI, there could only be 1 assessment per opportunity. Can you please help us figure this out?