The query validation should be added to the cb_email field. [value] is the value the user input into the cb_email field. Your current query is throwing a fatal error because cb_email does not exist in _users and can never work. So you need to check it against the _users email column as follows.

Wait, why are you trying to use a second field to verify the primary email address field? That functionality is built into every email field and turned on under the validation parameters. That query validation only works on the field you've directly configured it on. It takes the value the user has supplied to the field you've configured the query validation on and sends it through the query as per your conditions. You've it then set to be valid if it returns no results.

Ah, I understand what you're trying to do. You want your custom email field to be verified against the primary email address field to ensure it's a completely different email address. Sorry, misunderstood what you were trying to do. No there's no validation rules for doing that.