Hi,
process will follow as below
IW51-->RAS (repair order)-->Inbound delivery-->Service order (IW31)-->DP90--->billing
So if you are using Service item, then RAS will have these item categories
IRRS- Service item
IRRE-- InBound delivery will be created
IRRP- Service order will be created
Check if you have IRRP item category or not
regards,
santosh