Как исправить ошибку SQL-состояние: 42830

ОШИБКА: в целевой внешней таблице "sells" нет ограничения уникальности, соответствующего данным ключам SQL-состояние: 42830

(
    id serial PRIMARY KEY NOT NULL,
    shop_name varchar(30) NOT NULL,
    shop_adress VARCHAR(30) NOT NULL,
    shop_dir VARCHAR(50) NOT NULL,
    shop_tel char(11) NOT NULL
);
CREATE TABLE sells
(
    id serial PRIMARY KEY NOT NULL,
    product int NOT NULL,
    quantity INT NOT NULL,
    sell_date INT NOT NULL,
    shop int REFERENCES shops(id)
);
CREATE TABLE seller
(
    id serial PRIMARY KEY NOT NULL,
    seller_name VARCHAR(50) NOT NULL,
    seller_address VARCHAR(30) NOT NULL
);
CREATE TABLE good
(
    id serial PRIMARY KEY REFERENCES sells(product) NOT NULL,
    category int NOT NULL,
    good_name VARCHAR(30) NOT NULL,
    quant_box INT NOT NULL,
    price INT NOT NULL,
    sellers int REFERENCES seller(id),
    staffs int NOT NULL
);
CREATE TABLE staff
(
    id serial PRIMARY KEY REFERENCES goods(staffs) NOT NULL,
    staff_name VARCHAR(30) NOT NULL,
    staff_address VARCHAR(30) NOT NULL,
    staff_date int NOT NULL,
    staff_exp int NOT NULL,
    staff_lvl int NOT NULL
);
CREATE TABLE category
(
    id serial PRIMARY KEY REFERENCES goods(category) NOT NULL,
    cat_name VARCHAR(30) NOT NULL
);```