Add-ons can be applied to variable products, but can only be defined at the product level. Example: You cannot have an add-on for a specific variation of a variable product — for a blue shirt only. The add on would be created on the product as a whole.
Creating an add on does not make the product a variable product. You can have a simple product with add ons or a variable product with add ons, but the main product price must be added either at the top of the product screen on a Simple product or in Attribute & Variations section on a Variable product. The prices added next to each multiple choice is not the product price, it is an additional amount to be charged on top of the product price.