A card can have more than one type. For example, an artifact creature in your graveyard counts as both types for Lucid Dreams' effect.

Lucid Dreams is still on the stack and not in your graveyard as you calculate the value of X.

Card types that can appear on cards in a graveyard include artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Vedalken and Wizard are subtypes, not card types.