This has been an issue for the whole year the Lunar island has been available. While carrying the Altar Base and hovering over a celestial fissure there is no option to socket. I have even carried it to every single fissure on the island and no option appears. I have tried with the rest of the pieces of the altar as well to no avail. I have invited friends to try on my games as well and they had the same issue, could not socket any piece of the altar.
Mine Celestial Altar base on the lunar island. Carry the base to any celestial fissure. Options available are 'Examine celestial fissure' and 'Walk to celestial fissure'. Socketing is never an option.