Sylveon is flying type.
The Evolution Types on the right side of the chart are all super effective against their across types on the left.

Fire (Flareon) is super effective against Ice (Glaceon).
Dark (Umbreon) is super effective against Psychic (Espeon).
Grass (Leafeon) is super effective against Water (Vaporeon).

If this trend is accurate, then whatever Sylveon’s Type is will be either super effective against Electric or Electric will be super effective against it. The only possible type that is super effective against Electric is Ground (because it’s unused by other Eevee Forms). Based on Sylveon’s design, weight, and so on; I think it could be safe to assume that it isn’t Ground Type (I could be wrong, but that would be a weird Ground Type Pokemon). That leaves us with Flying Type:

Electric (Jolteon) is super effective against Flying (Sylveon).
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Name Argument
Sylveon’s name seems to indicate that it’s based on a flying creature. So, let’s take a look at some of it’s names across a few languages:

Sylveon (English, Spanish, and Italian name)
Nymphali (French name)
Nymphia (Japanese name)
(originally thought to be Ninfia, but “ ニンフィア” can easily be translated to Nymphia)

Let’s start off by taking a look at the English, Spanish, and Italian names. The “Sylv” part of the name seems to refer to a Sylph. In the occult philosophy of Paracelsus, a Sylph is a class of imaginary beings assumed to inhabit the air (usually depicted as women). If the name is actually based off of a Sylph, then being Flying type would make good sense.