Updating field ELIKZ EKPO on a CONTRACT(me33k) - how ?

I am updating field ELIKZ EKPO on a CONTRACT(me33k) using BAPI: BAPI_CONTRACT_CHANGE in my report program.

This BAPi give a message 'Change ELIKZ could not be effected'

MEssage ID of this message is I ME 664.

So, Is there any otherbapi or other solution to update this field.

Your help is highly appreciated.

Thanks in ADV.