Почему Create table не сочетается с alter?

CREATE TABLE EMPLOYEES (
    EMPLOYEE_ID NUMBER,
    FIRSTNAME VARCHAR(20),
    LASTNAME VARCHAR(20),
    PATRONYMIC VARCHAR(20),
    BIRTHDATE DATE,
    MOBILENUM NUMBER(8) NOT NULL UNIQUE,
    HOMENUM NUMBER(8) NOT NULL UNIQUE,
    CONSTRAINT FIO_BITH UNIQUE(FIRSTNAME,LASTNAME,PATRONYMIC,BIRTHDATE) 
);

ALTER TABLE EMPLOYEES
    ADD CONSTRAINT EMPLOYEES_PK PRIMARY KEY(EMPLOYEE_ID);

ALTER TABLE EMPLOYEES
    ADD CONSTRAINT FK_OBJECT
    FOREIGN KEY(OBJECT_ID)
    REFERENCES OBJECTS(OBJECT_ID);

CREATE TABLE OBJECTS (
    OBJECT_ID NUMBER,
    OBJECT_NAME VARCHAR(20),
    OBJECT_SQUARE NUMBER(4),
    OBJECT_COST NUMBER(6),
    CONSTRAINT CHECK_COST_SQUARE
        CHECK (OBJECT_COST > 0 AND OBJECT_SQUARE > 0)
);

ALTER TABLE OBJECTS
    ADD CONSTRAINT OBECTS_PK PRIMARY KEY(OBJECT_ID);