Monark is a game that can be a bit confusing when it comes to puzzles. If you don't look in exactly the right place, then you will be left trying to figure out a puzzle for hours. This especially becomes true when you first enter the Year two build where you meet Ryotaro Date.
Upon entering the year two building, you will find a room near Vanitas that's locked. This is where the phones that lead to the first ideal are held, but a young idol is currently locked inside. Once you speak with her, head all the way to the other end of the hallway, and go into the last open room.
Once in the room, head to the back. You will see a note and a safe. The code for the safe is 0623, the date the idol group got together. Once you get the photo out of the safe, head back to the locked room and choose the new dialouge option to show the photo to the idol.
She will leave the room, and you will now be able to gain access to the first ideal.
For more puzzles solutions, check out our Monark section on Pro Game Guides!