You need to be more specific. Is this when creating the function? If so, run the revised code below show the exact errors will be showed. Are you running this from SQL Plus?

>Mark,
>now it gives me an "Invalid Character"

CREATE OR REPLACE FUNCTION FNext
     RETURN NUMBER
   IS
     Count1 NUMBER(10);
  BEGIN
    SELECT PersonSN.nextval INTO Count1 FROM dual;
    RETURN Count1;
END;
/
SHOW ERRORS