Complete the following quests to get the Marble Pit:
Little Paradise
Flight of Fantasy
Flower Seller
Find 3 Picks
Marble Rush

You also need to fully complete the quest chains that unlock Sun Tiles and the Frozen Rainbow. If you still haven't unlocked these resources, complete the required quests first.
If you've completed all the quests, yet the Marble Pit is still locked, contact our support team.