Why can't I get Umbreon?

Reasons it might have failed:
You didn't get 2 candy drops. (Seems unlikely since you walked 20km)
It wasn't your buddy when you evolved it.
You had bad GPS location when pressing the evolve button.

Dude, you need to chill before you turn out like You Know Who.

I feel your pain though. Lost GPS on Larvitar Day and my level 30 12 atk Pupitar ended up with Bite.

It's your fault. You made one of the mistakes listed by housunkannatin. Swallow your rage and frustration and the next time pay more attention to all these variables that you just learnt you should have considered.