On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough description of the Taking Root mission.
Master smith Ivaldi needs the root of a mountain to forge the magic cord, Gleipnir.
Collect the root of a mountain
You can find a clue regarding where to find this ingredient by reading a note in your inventory' story items section.
Go to the Utangard Chams, the place indicated in the picture above, and climb to the top of the mountain there. If you already have this synchronization point discovered, fast travel to that location.
You will find the needed ingredient on the rock shelf just below the sync point. Pull out a torch and set fire to the area around the root of the mountain.
When the fire burns out, pick up the root, stand at the spot shown in the image above, and drop the item into the water.
Jump down, walk up to the item and interact with it. The task is accomplished!