Intercompany WHT Certificate not generating

Summary

Intercompany WHT Certificate not generating

Content

WHT Certificate not generating for Intercompany AP Invoice.
Created TAX Rule by Account number and WHT Tax calculating correctly after Payment.
Product Category field is Grayed out in Invoice Line level (This field is key for generating withholding tax generation in Thailand).
Payment made (Regional information updated).
Submit Withholding tax certificate job.
Certificate is not generated.

Just want to understand two points here:
Why Product Category field is grayed out?
How to modify WHT tax code which is derived by Tax Rules? Withholding tax calculation point at "Payment" Level so this field is not visible.