Professor Mirror will give you requests to complete the ‘Where it Snacks and Snoozes’ request. The request is to take a picture of a pokemon that is difficult to find but sleep near Torterra at night.
Load into Park(Night) in Florio Nature Park and head towards the lake area where a lot of Torterra can be found sleeping. You can find Sylveon can asleep next to a Torterra at the end of the lake area. It is a bright color pokemon in contrast to the black of the night.