UPDATE

SET [col] = [col]

Seriously? that is the trigger body? updating the column to itself, with referencing the INSERTED virtual table?

Remember we are not in your cube looking over your shoulder; we can only help you if you help us by providing the complete, real update command.

there's got to be much much more than you are telling us, because base don what I've seen so far, it seems you are trying to do something like this:

UPDATE PROD

SET Product_Name_Expiry = getdate()

WHERE Product_Name_Expiry IS NULL

that is the most basic of UPDATE commands, and your trigger must be more complex than the code you pasted, otherwise you wouldn't need a trigger at all.