Eldritch Smite is Warlock slot only

Only warlock spell slots from the Pact Magic feature can be expended to use Eldritch Smite; spell slots from the Spellcasting feature do not work with it.

An unofficial tweet by rules designer Jeremy Crawford in November 2017 confirms that this was intentional (unlike the paladin's Divine Smite, which had similar wording initially but was changed in errata later to work with any spell slot):

Talking about Eldritch Smite; can you only use Warlock spell slots to power it, or can you use any slot (à la Divine Smite)? The RAW on it specifies Warlock slots.

 

Eldritch Smite works with warlock spell slots only—the ones you get from Pact Magic.

Eldritch Smite is Warlock slot only

Only warlock spell slots from the Pact Magic feature can be expended to use Eldritch Smite; spell slots from the Spellcasting feature do not work with it.

An unofficial tweet by rules designer Jeremy Crawford in November 2017 confirms that this was intentional (unlike the paladin's Divine Smite, which had similar wording initially but was changed in errata later to work with any spell slot):

Talking about Eldritch Smite; can you only use Warlock spell slots to power it, or can you use any slot (à la Divine Smite)? The RAW on it specifies Warlock slots.

Eldritch Smite works with warlock spell slots only—the ones you get from Pact Magic.

Eldritch Smite is Warlock slot only

A tweet by Jeremey E. Crawford confirms this was intentional, unlike Divine Strike which had similar wording initially, but was erratad later.

Eldritch Smite is Warlock slot only

Only warlock spell slots from the Pact Magic feature can be expended to use Eldritch Smite; spell slots from the Spellcasting feature do not work with it.

An unofficial tweet by rules designer Jeremy Crawford in November 2017 confirms that this was intentional (unlike the paladin's Divine Smite, which had similar wording initially but was changed in errata later to work with any spell slot):

Talking about Eldritch Smite; can you only use Warlock spell slots to power it, or can you use any slot (à la Divine Smite)? The RAW on it specifies Warlock slots.

Eldritch Smite works with warlock spell slots only—the ones you get from Pact Magic.

You can use any slot for Divine Smite

PHB errata:

Divine Smite (p. 85). You can expend any spell slot, not just a paladin spell slot.

Eldritch Smite is Warlock slot only

A tweet by Jeremy E. Crawford confirms it was intentional to limit Eldritch Smite to warlock slots.

Eldritch Smite is Warlock slot only

A tweet by Jeremey E. Crawford confirms this was intentional, unlike Divine Strike which had similar wording initially, but was erratad later.