How to prevent users from submitting req/po when category is on an ASL?

Summary:

A requirement exists to prevent users from creating a requisition or purchase order using a category when the supplier is not approved in an ASL.

NOTE: BTW, this works if I use an item in the ASL, but it does not work if I use a category alone.

Content (please ensure you mask any confidential information):

Approved Supplier List is created for supplier ABC, for category MISC (No Item)

The user creates a PO for Supplier DEF using the category MISC.

The system allows the PO to be submitted for approval.

Expectation: The user is prevented from submitting the PO with a message indicating the supplier is not approved.