I walked 20 km with feebas in Pokemon go and I still didn't get 100 candies. Why?
Solution 1:
That's not how this works.
Feebas doesn't give you 100 candies for walking it 20km. Walking 20km is a requirement to be able to evolve the Feebas, as well as having 100 candies.
You'll need to get 100 candies yourself, in the same way you'd get them for any pokemon. You could walk more with Feebas, use rare candy, and/or catch more Feebases.