While evolving Eevee in Pokemon go I am getting an error?

If you were attempting to evolve a Party Hat Eevee from the current Pokémon Day event, this seems to be a bug related to it. See for example this Reddit thread reporting the same thing (with Linnea).

This error message typically points to a general network error when contacting the Pokémon Go's servers due to either a bad connection or something happening on Niantic's end.

Restarting the app and reloading Pokémon Go is a good place to start. After you do this, one of two things could happen:

• It still evolved: The network blip caused the evolution animation to not appear, thus giving the appearance it didn't evolve. The evolution still occurred in the background, just not visually.

• It didn't evolve: The network blip caused an unexpected error when attempting to evolve. It this persists after restarting the app (or getting a better connection), then it's likely something on Niantic's end. You can either wait it out, or contact support