In the case of an existing product, under certain circumstances additional variants can be added. What is important here is that it is your own product and not one of the templates.
If you have your own products, you can edit them under the client level under "Catalog -> Products -> [Your product] ->" call up the detail page and select "Variants" from the area with the button "add" to create a new variant.
Currently, only one variant can be added at a time, since the system compares all relevant data with the existing data and checks it for possible error.
After the new variant has been created in production, the product can be in the shop administration can be transferred to new shops or in the case of existing products can be synchronized in existing shops.
You can find more information here: Synchronize