This scene is triggered near the end of the current version of the game, meaning depending on where you are in the game, you may still have a lot to do before you are able to get the scene.
Anyhow, if you check the strategy guide, it'll be under School quest part 2 (do a search for " Fake Tinymon Card" and you will find it). Good luck
Time: 08:00 (Weekday)
Visit Sarah in her Bedroom to go to School together, then meet her on the Rooftop
After Recess/Break talk to a student in the Main Area (1F)
Then go talk to the girl to the 2F Restroom
Talk to her again to buy the Fake Tinymon Card (repeatable)
Now go to the Rooftop, ignore Sarah and go in the Vent (repeatable)
I had a very similar problem last week and my issue was I didnt go through the mafia mission tutorial before getting to that stage where you are now and it bugged out that mission hence the black screen. If its the same problem I had then to fix it you'll need to have a ealier save and go to your chosen mafia and do the mission tutorial before doing that Lucy quest. Good luck