@LES ken difficult to say without knowing more on what type of opertaions are on the Shop Order. I would anyways venture a guess .

Probably the shop order contains an outside processing operation and the system tries to create a Purchase Req / PO for the supplier? → Probably there’s no default tax code on the Supplier for Purchase Part of that specific outside processing material? And since you have the supplier tax code validation enabled at company level you’re getting the error.

Anyways the error message comes from the Company setup → Supplier Tax Code Validation