Configurable products are products with different attributes like colors, sizes, brands, or much more. It lets you offer variety to your users.
Read this elaborated article to know how you can add the configurable product to your store.
If the Design It button is not showing for the configurable product it means you need to configure the image sides and design area.
Admin needs to set the image side manually, for all the associated product images. The design area of the parent product will automatically be applied to the child product image. However, priority will be given to the design area of the child product.
Add the image side and create the design area of the product or else it will not be listed in the product options section. Read this article to know how you can add the design area and image sides of the product.