It will be unlocked after the completion of the 'Eternal Blue Sky' main tale at the end of which Yuna will inform you that your uncle wants to meet you.
This is the last main tale summarizing Jin's relationship with his uncle. In order to start this tale you must first get to Kamiagata and then find a freezing musician trying to light a campfire in the camp located north of Jogaku Temple. To see the full list of tales, follow this link - List of Ghost of Tsushima Tales. Here is a list of recently added or modified tales.
