Aha I know that's a bug because you finished Isaac's storyline before you enough Linda's relationship. Only way to unlock solo route is play the game again and choose lie to Isaac.
Just wait when you enough Linda and Oliva's relationship
And the sidequest will randomly when you visit her at the morning, it will take you to peek her in the bathroom.
After you finished Lily's mission in GreenBerg's mansion, meet her in that place at the morning.
