On the following page of the Assassin's Creed Valhalla guide, you will find a walkthrough of the Last Leaf of Fall side quest.
Award for completion: 1200 XP.
In the place marked in the picture above, you will meet a child, Mae, who apparently cares about the fate of a particular leaf on the tree.
You will find out from the conversation that her father went to Lincoln to find a job and will return before all the leaves fall off the tree. Unfortunately, 6 weeks have passed and he still has not returned. During the conversation, you can choose one of 3 dialogue options.
If you don't want the girl to hate you, it is better to choose 1 or 3 options. Nevertheless, no matter which option you decide on, the girl will return to observe the leaf.
The only solution to this side quest is to cause the leaf to fall. Hit the tree and after a while, the leaf will be on the ground and the girl will leave the tree and hide in the house.
You can pick up the leaf and take it to the cottage and then put it on the stool right next to the girl's bed but this does not in any way affect the reward you will receive for doing this quest.