If your product has variants and you want to add Size or Color, Click on Add Variants at the bottom while adding a new product or editing the details for a product.
To add Size, click on Add Size button. Enter size name, its MRP, and selling price. If you want to delete any size, click on the Delete button to remove it.
If the product has different color options, click on Add Color and select the preferred color from the colors shown.
If the color is not matching with the product, click on Custom to select the desired color according to the product.
Once done, click on Save and continue button at the bottom. Your variant details will be added to the product.