The default variant is always just the first one. A product cannot exist without a variant. So, if you want to create a default variant, what you're really saying is is that you want to create a product, with one variant. No obligation to create two or more and you need no options either. Note that in reality, when creating a product, it has no SKU. Only a variant has a SKU. Your setup shows you trying to set a SKU on a product which will almost certainly fail.