Etsy wants to make sure that all of the goods sold on their channel are handmade, crafts, or vintage good (except when the item is a supply or a craft tool). If someone else has made an item that you are trying to list on Etsy, you will have to mark this as a supplied product.
To mark this product as a supplied product, navigate to Listings-->Queued Listings-->Listing Details (binocular icon). Check the box next to "Is Supply?", and click the blue "Save" button.
Once you have clicked the blue "Save" button, you will be able to continue listing your product.