Espeon can learn Sing, but only when evolved from a special event Eevee that was distributed in Japan during 2013. You cannot get an Espeon with Sing though ordinary circumstances.
(According to Bulbapedia, you received a voucher to obtain this Eevee if you purchased a song by the Japanese band Ikimono-gakari, which is definitely one of the franchise's stranger event distributions!)
Coverage of event moves on PokemonDB's and Serebii's Pokedex pages is a bit spotty, so when you find oddities like this, check Bulbapedia because it's the most complete resource to find an explanation.
