Oracle SQL trigger on update of column

create or replace
  TRIGGER ADD_CREATE_DT 
  after UPDATE of approved ON articles 
  for each row
  BEGIN
    IF :NEW.approved = 'Y' THEN
      :new.create_dt := sysdate;
    END IF;
  END;

Use the WHEN clause:

create or replace
  TRIGGER ADD_CREATE_DT 
  after UPDATE of approved ON articles 
  for each row
  when (new.approved = 'Y')
  BEGIN
  :new.create_dt := sysdate;
  END;

Or use IF:

create or replace
  TRIGGER ADD_CREATE_DT 
  after UPDATE of approved ON articles 
  for each row
  BEGIN
  if :new.approved = 'Y' then
   :new.create_dt := sysdate;
  end if;
  END;

In this case, WHEN is more appropriate and efficient.

I don't know What version of Oracle do you use? In Oracle 10g I got the following error:

ORA-04084: cannot change NEW values for this trigger type
04084. 00000 -  "cannot change NEW values for this trigger type"
*Cause:    New trigger variables can only be changed in before row
           insert or update triggers.
*Action:   Change the trigger type or remove the variable reference.

It does not allow to change the field on AFTER triggers.