Hi,
Then, it probably means that the email address had been used before for the creation of a Joomla user account, and the user_cms_id linked to the user account was not reset due to how the user account was deleted, circumventing the synchronization process of HikaShop.
Change the line:
$query = 'UPDATE `#__hikashop_user` AS hku LEFT JOIN `#__users` AS ju ON hku.`user_email`=ju.`email` SET hku.`user_cms_id`=ju.`id` WHERE hku.`user_cms_id`!=ju.`id`';
to:
$query = 'UPDATE `#__hikashop_user` AS hku LEFT JOIN `#__users` AS ju ON hku.`user_email`=ju.`email` SET hku.`user_cms_id`=IFNULL(ju.`id`, 0) WHERE hku.`user_cms_id`!=ju.`id` OR ju.`id` IS NULL';
in the file administrator/components/com_hikashop/helpers/database.php via FTP of a file manager and then click on the "check database" button of the Hikashop configuration page. It will reset the user_cms_id of that email address, so that you can proceed with the checkout as a guest with it.