You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'AND id_product = 0),
...' at line 2
UPDATE ps_cart_product SET rental_days =
(SELECT rental_days FROM ps_products_in_availability_check WHERE id_customer = AND id_product = 0),
start_date =
(SELECT start_date FROM ps_products_in_availability_check WHERE id_customer = AND id_product = 0),
svdays =
(SELECT svdays FROM ps_products_in_availability_check WHERE id_customer = AND id_product = 0),
end_date =
(SELECT end_date FROM ps_products_in_availability_check WHERE id_customer = AND id_product = 0)
WHERE id_product = 0 AND start_date IS NULL