why cannot assign new item category to sales doc type

Dear all,

I found an old post regarding "Milestone Billing" in this community and tried to follow the steps.

I could "Create New Item Category", however, I cannot complete "Assign Item Category".

I assume it cannot be assigned because

Item Category I created(ZTAS, ZTAD, ZTAS) are not found in the selection list.

How can I assign Item Category I created(ZTAS, ZTAD, ZTAS) to Sales Document Type?

These are steps I've tried..

.Create New Item Category, say ZTAN - Copy of TAN (For delivery Related) / ZTAD - Copy of TAD (For Order realted) / ZTAS - copy of TAS (For Third Party)

Maintain as under (others remaining same)

Billing Relevance: I

Billing Plan Type: 01

IMG > Sales and Distribution > Sales > Sales Documents > Sales Document Item > Define Item Category

.Assign Item Category, for ex (assuming Usage & higher level item category as blank):

Sales Document Type | Item Category Group | Default Item Category

ZOR | NORM | ZTAN

ZOR | DEIN | ZTAD

ZOR | BANS | ZTAS

IMG > Sales and Distribution > Sales > Sales Documents > Sales Document Item > Assign Item Category

thank you for your support in advance!

abby