The code doesn't work because it's wrong. Try this:

CREATE TABLE EMPLOYEES
(
EMPID NUMBER(10),
LASTNAME VARCHAR2(25) NOT NULL,
FIRSTNAME VARCHAR2(25) NOT NULL,
CONSTRAINT PK_EMPLOYEES_01
PRIMARY KEY (EMPID) DISABLE
);

If the Handbook you are referring to is the one from Oracle Press, be advised that it is notoriously flawed.