Evolving Feebas [duplicate]
It is not an or it is an and. You need to have 100 candies and have had to have walked 20km with your Feebas before you can make that final plunge and evolve into a Milotic.
Edit: You must have 20km distance on the Feebas you want to evolve i.e. A Feebas' distance bar must be full before evolving.