If you're not able to see the product variants when you create your customizable products in Zakeke, please read below the guide that refers to your eCommerce platform:
BigCommerce
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.
OpenCart
In OpenCart's admin, you have to set up your options in your products. To create options, you have to go to Catalog > Options. Then, proceed to your products' configurations at Catalog > Products > Edit for whichever product you wish to configure. Under the 'Option' tab, you should be able to set the options you've created in Catalog > Options.