Mammoth Galleons cannot be created at your Shipyard.
You need to transform a ship you already own, using Premium Ship Tier Up Ticket.
※ When you choose a ship to transform, the chosen ship will disappear.
※ Ship levels will be inherited but stats you get from Upgrades / Calypso's Blessing will be paid back in Silver and reflected in Mammoth Galleon stats.
※ You can Craft Premium Ship Tier Up Ticket at Shipyard using Premium Ship Blueprints.