I've walked 10km, not just gotten 2 Candies, and it's nighttime, but it still has the ? icon rather than the Umbreon silhouette on the Evolve button.
I've also tried renaming it Tamao, and whilst the button then becomes Umbreon's silhouette, when I try and evolve it, it says to 'finish a challenge with this buddy'. I'm the only one who uses my GO account, and I haven't used the Tamao trick before.
In case it's relevant, it's Dynamax and traded because all the other Eevees I have (including the one you encounter in the research to evolve) are anniversary ones that can't evolve.
