create or replace
FUNCTION GRADECOM (  GRADE IN VARCHAR2  ) 
RETURN VARCHAR2 IS 
COMMENTS VARCHAR2(30); 
BEGIN 
    CASE GRADE
          when 'A' then dbms_output.put_line('Excellent');
          when 'B' then dbms_output.put_line('Very good');
          when 'C' then dbms_output.put_line('Well done');
          when 'D' then dbms_output.put_line('You passed');
          when 'F' then dbms_output.put_line('Better try again');
          else dbms_output.put_line('No such grade');
       END CASE;
      RETURN NULL;
END GRADECOM;

select gradecom('A') from dual;

Output:
null