Hi..
If you activate material ledger, after occurring mm transaction you can’t
Release standard cost in current month.
To release standard costing for a material, you should cancel mm transaction related to a material in current month and change material ledger’s status into “10” using ZCKML_RESET_STATUS_OF_PERIOD which is supplied by note 574930.
If you want to solve your problem related to billing error due to no standard cost, refer to scn site : http://scn.sap.com/thread/3402205.