creating the WBS using bapis

i am creating the WBS using thses bapis

1. BAPI_PS_INITIALIZATION

2. BAPI_BUS2001_CREATE

3. BAPI_PS_PRECOMMIT

4. BAPI_TRANSACTION_COMMIT/BAPI_TRANSACTION_ROLLBACK

but i am getting the error

Field COMPANY_CODE/Company code: Changing the contents of the field is not permitted

Field BUSINESS_AREA/Business area: Changing the contents of the field is not permitted

Field WBS_ACCOUNT_ASSIGNMENT_ELEMENT/Acct asst elem.: Changing the contents of the field is not permitted

Field OBJECTCLASS/Object Class: Changing the contents of the field is not permitted

Please help to resolve this

kanishak