Hi, I had a problem with these tables

CREATE TABLE GM_GIACENZA (
IDREC INTEGER NOT NULL,
DATA TIMESTAMP NOT NULL,
ARTICOLO CODICE_ARTICOLO /* CODICE_ARTICOLO = VARCHAR(20)
NOT NULL */,
GIACENZA FLOAT NOT NULL,
LOTTO VARCHAR(6) DEFAULT '',
DATA_INSERIMENTO TIMESTAMP DEFAULT 'TODAY' NOT NULL
);

ALTER TABLE GM_GIACENZA ADD CONSTRAINT PK_GM_GIACENZA PRIMARY KEY (IDREC);

CREATE UNIQUE INDEX GM_GIACENZA_IDX1 ON GM_GIACENZA (DATA, ARTICOLO,
LOTTO);

CREATE TABLE ARTICOAN (
PROGRESS INTEGER NOT NULL,
STATO VARCHAR(1) NOT NULL,
NDITTA DITTA NOT NULL,
DUMMY01 VARCHAR(2) NOT NULL,
CODREF VARCHAR(20) NOT NULL,
DESCR VARCHAR(40) NOT NULL,
DESCR2 VARCHAR(40) NOT NULL,
DUMMY02 VARCHAR(10) NOT NULL,
GRART VARCHAR(4) NOT NULL,
DUMMY03 VARCHAR(16) NOT NULL,
GRMERC VARCHAR(2) NOT NULL,
DUMMY04 VARCHAR(1) NOT NULL,
FITTIZIO VARCHAR(1) NOT NULL,
PRFINITO VARCHAR(1) NOT NULL,
SEMILAV VARCHAR(1) NOT NULL,
MATPRIMA VARCHAR(1) NOT NULL,
GESTIONE VARCHAR(1) NOT NULL,
DUMMY05 VARCHAR(9) NOT NULL,
UNIMIS VARCHAR(2) NOT NULL,
DUMMY06 VARCHAR(22) NOT NULL,
PESOUNIT DECIMAL(9,3) NOT NULL,
ARAN_MISURA_CONFEZIONE VARCHAR(2) NOT NULL,
ARAN_PEZZI_CONFEZIONE DECIMAL(9,5) NOT NULL,
ARAN_CONV_STOCC_CONFEZ DECIMAL(9,4) NOT NULL,
ARAN_LINEA VARCHAR(3) NOT NULL,
ARAN_BARCODE VARCHAR(40) NOT NULL,
ARAN_PRESIDI_SANITARI VARCHAR(1) NOT NULL,
ARAN_CODICE_PRESIDIO VARCHAR(6) NOT NULL,
ARAN_MISURA_PRESIDIO_SAN VARCHAR(2) NOT NULL,
ARAN_CONV_STOCC_PRESIDIO_SAN DECIMAL(9,4) NOT NULL,
ARAN_REGISTRO_SEMENTI VARCHAR(1) NOT NULL,
DUMMY_07 VARCHAR(9) NOT NULL,
NOMENCLA VARCHAR(8) NOT NULL,
DUMMY08 VARCHAR(32) NOT NULL,
CLASSE VARCHAR(8) NOT NULL,
DUMMY09 VARCHAR(312) NOT NULL,
ARAN_CONTENITORE VARCHAR(1) DEFAULT '' NOT NULL,
DUMMY10 VARCHAR(11),
IDCOMMENTO INTEGER NOT NULL,
UTENTE VARCHAR(10) NOT NULL,
DATAMOD TIMESTAMP NOT NULL,
ORAMOD INTEGER NOT NULL
);

ALTER TABLE ARTICOAN ADD CONSTRAINT PK_ARTICOAN PRIMARY KEY (PROGRESS);

CREATE INDEX ARTICOAN_IDX01 ON ARTICOAN (NDITTA, CODREF);
CREATE INDEX ARTICOAN_IDX02 ON ARTICOAN (DUMMY01, NDITTA, CODREF);
CREATE INDEX ARTICOAN_IDX03 ON ARTICOAN (NDITTA, DESCR, CODREF);
CREATE INDEX ARTICOAN_IDX04 ON ARTICOAN (NDITTA, GRMERC, CODREF);
CREATE INDEX ARTICOAN_IDX05 ON ARTICOAN (NDITTA, DUMMY02, CODREF);
CREATE INDEX ARTICOAN_IDX06 ON ARTICOAN (NDITTA, CLASSE, DESCR, CODREF);
CREATE INDEX ARTICOAN_IDX07 ON ARTICOAN (NDITTA, DUMMY05, CODREF);
CREATE INDEX ARTICOAN_IDX08 ON ARTICOAN (NDITTA, GESTIONE, CODREF);
CREATE INDEX ARTICOAN_IDX09 ON ARTICOAN (NDITTA, DUMMY05, DESCR, CODREF);
CREATE INDEX ARTICOAN_IDX10 ON ARTICOAN (NDITTA, GRART, CODREF);
CREATE UNIQUE INDEX ARTICOAN_IDX11 ON ARTICOAN (CODREF);

I would like to create a constraint for GM_GIACENZA table on field
ARTICOLO using ARTICOAN table and field CODREF but when I try to add it..

alter table GM_GIACENZA
add constraint FK_GM_GIACENZA_1
foreign key (ARTICOLO)
references ARTICOAN(CODREF)
on delete CASCADE

...I had this error:

This operation is not defined for system tables.
unsuccessful metadata update.
could not find UNIQUE or PRIMARY KEY constraint in table ARTICOAN with
specified columns.

As you can see, there's is the ARTICOAN_IDX11 index which contain the
rquired column and it is set to unique...why it doesn't work ? Can you
help me to find what's wrong ?

Thanks