The reason you're getting this error message is that the product has variations (for example a t-shirt coming in several colors/sizes). You can't add to cart since you did not choose a specific combination of these variations.
You need to add a variation chooser to your item details. You can try the ItemVariationsView
module.
This view will display all the possible variations to the user. By default, it uses image thumbnails, so you'll have to prepare an image for every option. If you don't want to mess around with all these images, you can add a DefaultRenderTypeOverride
and make it a Select
. This will make the variations appear as a combo-box.