Proforma Invoice for Service Material with reference to Order

Dear Experts,

I want to create the proforma Invoice for Service Material w.r.to Order.

I have maintained the copy controls in the VTAF and also maintained the item category as "TAD" at Item level.

I have tested by maintained the Billing relevance in Item category"TAD" as "D","Blank","L" etc.

I have also maintained the billing type in sales document type as "F5" to pick the billing type as F5 default.

Still system is throwing an error stating " ITEM CATEGORY TAD IS NOT RELEVANT FOR PROFORMA INVOICE".

Kindly help me where I am doing mistake.

Regards,

SAQ