Error with my trigger. What should I do?
Oracle doesn't have an inserted (or a deleted or an updated) table.  That's something that SQL Server provides in a trigger.  The Oracle substring function is also substr not substring.  

My guess is that you just want

IF :new.fsa IS NULL
THEN
  :new.fsa := SUBSTR(:new.POSTAL_CODE, 0, 3);
END IF;