You don't have any characteristic on your option product. That's why you don't have a label for the dropdown.
We say in the documentation above to create a characteristic for the option product and add that characteristic to the option:
Let's take the example of the creation of the RAM option :
First, we made one characteristic called RAM quantity with the three values.
Second, we created one product that we called RAM with that characteristic added to it.
Third, we clicked on the "manage variants" and set the name of each variant and its price.
Finally, we added the RAM product as an option of the desktop computer product on the desktop computer edition screen.
