problema con FK

[oscar2h]

CREATE TABLE clientes
(
kx_rut_cliente character varying(13) NOT NULL,
x_nombre_c character varying(30),
x_apellido_c character varying(30),
n_fono_c integer,
CONSTRAINT pk_11 PRIMARY KEY (kx_rut_cliente)
)

CREATE TABLE reservas
(
kn_id_reserva integer NOT NULL,
kx_rut_cliente character varying(13),
n_cantidad_personas integer,
CONSTRAINT reservas_pkey PRIMARY KEY (kn_id_reserva)
)

esas son las tablas gracias