Sales BOM set up problem

Dear experts,

I got a problem need your help.

Business requirement: Our commpany sold air conditioning. All of our products included 2 Components - the indoor unite A, and out door unit B. In MM, A & B are purchased and managed in stock separately. But from SD, we sold it by product A+B, and the sales price is for the combination of A+B, no separate sales price for A or B. So we have 2 items in SO, but only the total price for A+B. this why i am thinking to working on it with Sales BOM .

Configuration :

1. Set up the item assignment :

ZOR + ERLA = ZTAQ
ZOR + NORM + ZTAQ = ZTAE.

2. Set up the Item category group of Material master data

C: ERLA

A: NORM

B: NORM

3. Set up the sales BOM

for Material C, usage = 5(Sales)

with 2 items: A , item category (L - Stock), 1

B, item category (L - Stock), 1

4. Create Sales order with order type ZOR, after input the material C, order qty =1, SAP sent a warning message " 000001, Please Enter order quantity". then i Pressed ENTER, my SO show up the sales BOM, but without quantity.

Question: did i make any mistake as above ? why SAP warned me "No order quantity", even i had input quantity in the sale item level ??

thanks for your time.