If your store is built on WooCommerce platform, Zakeke can only recognize attributes that are present under Products > Attributes menu in the WP admin.
Attributes created "inline" at the product page level are not supported yet.
This could be the reason why you don't see the Variations step in Zakeke.
Our advice is to add attributes that you need in Products > Attributes and then add variations to the product using those attributes.
You have to create a SKU on BigCommerce for each variation that you want to see in Zakeke. Please refer to the BigCommerce documentation on how to create an SKU.