How Do I Do The Name Trick?

So, like everyone, I’m trying to get all the eevee-lutions. I know of the name trick, here I’ve named my shiny eevee ‘Sakura’ trying to get Espeon.

The issue is, it asks to complete a challenge. I searched how to find the challenge I need to complete on the buddy menu but to no avail.

If anyone could shed some light on this it would be greatly appreciated.

Sincerely, a fairly new Pokemon Go player!