I know, Lunala is the emblem of the moon but shouldn't it be psychic/dark type instead of a psychic/ghost type? please help me I am very confused.
Part of the reason is that the Dark type is associated with evil, and Game Freak may not want Lunala considered evil. Another reason is its origin. Here's what Bulbapedia says:
Lunala appears to be based on a bat with a Moon motif. In particular, it might reference the two known Hawaiian bat species, the still living Hawaiian hoary bat and the extinct Synemporion, or the ghost bat. It could also be based on Itzpapalotl, a skeletal Aztec goddess and star demon, often depicted with bladed bat wings; Camazotz, a Mayan bat god that was associated with the night, death, and sacrifice; Leutogi, a Samoan protector bat goddess; and Pe'a-pe'a-makawalu, an eight-eyed bat that Hawaiian demigod Maui snatched eyes from. This may also be a basis for Lunala's auxiliary eye in Full Moon phase.
Lunala appears to be based on a bat with a Moon motif. In particular, it might reference the two known Hawaiian bat species, the still living Hawaiian hoary bat and the extinct Synemporion, or the ghost bat.
It could also be based on Itzpapalotl, a skeletal Aztec goddess and star demon, often depicted with bladed bat wings; Camazotz, a Mayan bat god that was associated with the night, death, and sacrifice; Leutogi, a Samoan protector bat goddess; and Pe'a-pe'a-makawalu, an eight-eyed bat that Hawaiian demigod Maui snatched eyes from. This may also be a basis for Lunala's auxiliary eye in Full Moon phase.
Lunala is quite literally a Ghost bat, so it would obviously be a Ghost type. One of its potential inspirations comes from a god of death. Ghost types are dead Pokemon, so that is very likely another reason why.
Source
Hope I helped!
I think its because ghosts are more related to the night than dark because dark is associated with evil tactics and cheating/crimes rather than the dark itself. WHICH MAKES NO SENSE BECUASE POKEMON LIKE ABSOL AND UMBREON PROTECT PEOPLE LIKE WHAT THE FRICK GAMEFREAK. Its also why Dark is weak to Fairy and Fighting because Good beats Bad. Also lunala is based off of a bat and bats live in the dark and are noctornal so yea.