Hi,
Go to VF02 and check enter the intercompany billing doc number. Now go to Menu - Billing document - Release to accounting.
If there is error, system will show it in information bar. Double click on the message and check the details. If you are not able to understand, please post the error message with details.
You may refer the document http://scn.sap.com/docs/DOC-46122 for process flow.
Regards,
AKPT