I have my buddy Feebas and have walked 20km with it. Is it too late to evolve?
Is it too late for me to complete this special research?
Feebas with 20km and over 109 candies still will not evolve?
To evolve Feebas you are required to set it as your buddy and walk a minimum of 20km with it. Once you have completed this task, you can then freely evolve it.
The notification you are receiving about choosing to evolve it, is only in regards to the https://en.m.wikipedia.org/wiki/Pok%C3%A9mon_GO_Battle_League . There are "caps" of 1500 CP, and 2500 CP. Evolving your pokemon may automatically boost their CP beyond one of these caps, which is unavoidable.
There is nothing in regards to this choice being "too late", as this is standard practice for Feebas/Milotic evolution