Creating a required field to close issue ONLY on Parent Story

Hello,

I want to create a required field ONLY for parent stories. I DO NOT want the field to be required for Sub-Tasks. I have tried to follow 

https://confluence.atlassian.com/jirakb/how-to-make-a-field-required-before-closing-the-issue-1236928291.html

But Options - Validators is not available as a selection.