Hi Venu,
Actually Item Category Group :- ZSER (Service w/o Project) and ZSEP (Service with Project).
As per our case we need not do any delivery. We want to create Billing Documents from sales order directly (without doing any Outbound Delivery & Post Goods Issue).
So by mistake we selected ZSER item category group and assigned the same with Item Category Instead of ZSEP.
While directly Invoicing the SO system don's have any data to pick up for Project assigned to that SO.
Once i assigned the correct Item Category with right Item category group its get Invoiced.
Regards
Amitesh