Hi,

Here is what you need to do. Make sure that you don't have any other column defined as Not Null. Your Logic is against the Reality. Before Insert you are Updating the Column, while the purpose of the triiger is to have the Sequence value placed there. Right

Here is the Triger Code

CREATE OR REPLACE TRIGGER RF_PROD_BOM_INS_TRG
BEFORE INSERT ON RF_PRODUCT_BOM
FOR EACH ROW
Declare
V_SeqVal Number;
BEGIN
Select F_PROD_BOM_SEQ.NEXTVAL Into V_SeqVal From Dual;
:New.PBRFNBR = V_SeqVal;
END;

Hope this will help you.. Thanks