UPDATE [table] 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:
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.
help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!