Foreign Keys

I have this very simple practice database is 11g and I can't add a foreign key to a table. Maybe someone can tell me what I'm doing wrong.

I have table one

create table employee(

        fname varchar2(20) NOT NULL, minit char(1),
        lname varchar2(20) NOT NULL, ssn number(9) NOT NULL,
        bdate date NOT NULL,
        address varchar2(50) NOT NULL,
        sex char(1) NOT NULL,
        salary float(12) NOT NULL, superssn number(9),
        dno number(2)
        );

I set a primary key

alter table employee add constraint pk_employee primary key (ssn) /

I create table two

create table works_on(

                essn number(9),
                pno number(2), hours float(5) );

ALTER TABLE WORKS_ON ADD CONSTRAINT workson_essn FOREIGN KEY (ESSN) REFERENCES EMPLOYEE (SSN); SQL> _at_fk_workson_essn.sql ADD CONSTRAINT workson_essn

               *
ERROR at line 2:
ORA-02298: cannot validate (RUBEN.WORKSON_ESSN) - parent keys not found

what is the error?

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