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./classses/Product.php

find public function addAttribute.

change quantity = 0 to 1

public function addAttribute(
        $price,
        $weight,
        $unit_impact,
        $ecotax,
        $id_images,
        $reference,
        $ean13,
        $default,
        $location = null,
        $upc = null,
        $minimal_quantity = 1,
        array $id_shop_list = [],
        $available_date = null,
        $quantity = 1,
        $isbn = '',
        $low_stock_threshold = null,
        $low_stock_alert = false,
        $mpn = null
    )