You cannot invoke a validate rule from a data transform

From Help:

Where referenced

Rules of three types can reference validate rules:

• Flow action rules, to validate user input
• Activities, with the Obj-Validate method
• Correspondence rules, to validate user responses to prompts

To execute a validate rule during initial entry of a work item, create a validate rule named onAdd with a work type as the key part. The standard activity Work-.Add causes a validation rule with that name to execute. 

You can also create a validate rule called Validate that will run when a work item is updated.