You can purchase an AppleCare warranty plan only if you buy an Apple product refurbished and sold by the Apple Store

If you want to buy AppleCare warranty, make sure to filter products refurbished only by Apple Store when browsing RefurbMe. Then, select the device you are interested in and pick the one from the Apple Store:

After you click "Buy now", you will be redirected to the Apple Store ecommerce page, where you will be able to add an AppleCare plan to your order.