Validate Field Values: If Category is **** and DFF is null display error message in RSSP

Summary:

I am trying to create a Validation rule on a DFF in Requisition Lines.

If the Category Id / Category Name = XXXX and the DFF field is null then we should produce an error message - essentially making the DFF mandatory when specific categories have been used.

It is almost working but the error message is not displaying. Is anybody able to confirm why?

If I create a requisition for the category ID mentioned below and I leave the DFF null, clicking Add to Cart does not let me proceed, but no error message is presented. If I populate the DFF so it is not null or change the category to something else then I can add to cart, which confirms the validation is succeeding.