Forza Horizon 6 uses several map landmarks for Festival Playlist challenges, and Shikisai-no-Oka is one of the locations used in the game’s newest one, the Series 3: Italian Exotics. You need to visit it during the weekly Photo Challenge, Petal Power, where the goal is to take a picture of a Ferrari at the flower field.
Shikisai-no-Oka is located in the Hokubu region in Forza Horizon 6. If you have location markers enabled on your map, you can find the white-and-blue sign saying Shikisai-no-Oka. From there, you can either drive or fast travel there.