Shipping Director and error messages

Some of our products are now restricted from being sold within certain states.
So I created a category of CA and put it on the items that are restricted.

Is it possible to reference to Item SKU in the Description Expression of SD?

I want to show a message of "Cart contains items restricted in CA SKU123

That way if cart contains multiple items, shopper will know what the offending SKU is.

Yes, you can reference variables in the Description Expression, just be sure the final result is a String.  E.g.

"Cart contains items restricted in CA  " + Items.First(Product.HasCategory("CA Restricted")).Product.Sku

This assumes your ErrorExit condition already checks :
  State = "CA" and Items.Any(Product.HasCategory("CA Restricted"))
(Otherwise you will get an error when trying to access '.Product' when there aren't any)

Note, it's only the first offending product.  Listing all offending SKUs is a little trickier.  (Let us know if you need to)

(P.S.  If you reference your own variables, be sure you bracket them  - eg.  [MyVar]  )

Works like a charm!  Thank you!