push button - Screen enhancement for contract MM06E005

Hello,

I am using the enhancement MM06E005 to add custom fields on the subscreen. i have created a table control there and i am trying to use two push buttons, one is for adding entries to table control and another one is for deleting.

But the problem is , system is giving an error saying this (custom function codes) is not available in the table T168F. This is a table where all the function codes for the purchasing document(contract) is mentioned.

Any body knows how to go about this problem. Do we need to contact with SAP for this.

Thanks