In 2.3.1 I found that this code is compulsory. After 2.2.6 magento requires sku
compulsory. So this problem is comes. I face this problem and i follow this url.
https://github.com/magento/magento2/issues/19761
I put sku in my custom code. and my code is working fine. My problem is solved.
I wrote like this:
class CustomImport extends \Magento\ImportExport\Model\Import\Entity\AbstractEntity {
const SKU = 'sku';
const ADD1 = 'add1';
......
protected $validColumnNames = [
self::DESCRIPTION,
self::COMPANY,
self::SKU,
self::ADD1,
self::ADD2,
self::ADD3,
self::ADD4,
self::ADD5,
self::CITY,
self::STATE,
self::ZIP,
self::COUNTRY,
];
}