This is a duplicate of an older question that got two incorrect answers and then got closed. the original poster described the problem well so I'll just c/p what they said:
"When you attempt to add a variant to an existing product, it doesn't give you the same interface window that it did when you initially created the product. Instead of being able to enter multiple product variants separated by a comma, you have to make one new variant, save and close the window, and repeat that for every new variant you want to add. That's so difficult that's its easier to delete the product and make a new one which is absolutely ridiculous. Especially considering you can do it initially. I'm not here to delet my products I'm here to get them online and sold."
The option to add new variant types to an existing product does not allow you to add variants, it allows you to add 1 variant, which is obviously rather useless.
Is the only way to do this to delete the product and re-upload everything?