Getting Account-Determination related Error during Service Entry Approval

Dear Sir,

We have created a Service Order (Purchase Order) with Account-Assignment Category as "Q" and Item-Category as "D" . The In the Purchase Order , Account assignment given was a WBS Element and appropriate GL for expense booking was also mentioned .

Subsequently , we did Service Entry (ML81n) .

Now during the approval of Service Entry Sheet , we are getting following error...

"Account determination for entry 9000 KBS 0001 not possible" .

We request to kindly guide us as what steps need to be followed to resolve this error . We will be highly greatful for the given kind help pl .

With Regards

Sania