Like most Legend of Zelda games, you'll need to find the Master Sword before it can be added to the inventory in Tears of the Kingdom. While Link starts off with the blade of evil's bane in the prologue, it's deterioted by gloom from the decayed Ganondorf in The Depths below Hyrule Castle. A series of story events will lead to Link sending the Master Sword to Princess Zelda, and it may take some time before it finds its way back to Link.