Cannot Interact with Runecarver to turn in Legendary Memories

If you have Legendary Memories in your inventory, as long as you are in the Runecarver's Oubilette you should be able to consume the items and "teach" them to the Runecarver after a channel time.
If the Runecarver is missing or if you see the Primus in the Runecarver's space, try to continue the Zereth Mortis campaign if you have already started it. Completing quests such as Reality's Doorstep should place the Runecarver back to the proper phase.