Table Oracle PLSQL Tutorial

SQL>
SQL> CREATE TABLE employee (
  2  id                             number,
  3  name                           varchar(100),
  4  birth_date                     date,
  5  gender                         varchar2(30) );
Table created.
SQL>
SQL> CREATE TABLE employee_evaluation (
  2  id                             number,
  3  title                          varchar2(100),
  4  written_date                   date );
Table created.
SQL>
SQL> ALTER TABLE employee_evaluation ADD
  2  CONSTRAINT  employee_evaluation_fk1
  3  FOREIGN KEY        (id)
  4  REFERENCES  employee (id);
REFERENCES  employee (id)
                      *
ERROR at line 4:
ORA-02270: no matching unique or primary key for this column-list
SQL>
SQL> drop table employee cascade constraint;
Table dropped.
SQL>
SQL> drop table employee_evaluation cascade constraint;
Table dropped.
SQL>