MySQL - Error alter table foreign key

alter table tbl_cuentas

add CONSTRAINT FK_PersonOrder

FOREIGN KEY (identificacion) REFERENCES tbl_clientes(identificacion);

mysql> CREATE TABLE tbl_clientes (

    ->   identificacion INT NOT NULL,

    ->   nombreCliente VARCHAR(20),

    ->   PRIMARY KEY(identificacion)

    -> );

Query OK, 0 rows affected (0.30 sec)

mysql> CREATE TABLE tbl_cuentas (

    ->   cuenta INT,

    ->   identificacion INT,

    ->   PRIMARY KEY(cuenta)

    -> );

Query OK, 0 rows affected (0.22 sec)

mysql> ALTER TABLE tbl_cuentas

    -> ADD CONSTRAINT FK_PersonOrder

    -> FOREIGN KEY (identificacion)

    -> REFERENCES tbl_clientes(identificacion);

Query OK, 0 rows affected (0.73 sec)

Records: 0  Duplicates: 0  Warnings: 0