Can you please clarify what you mean by this? I’ve been trying to romance Shannon but I keep triggering Lana’s route instead on accident and I don’t know what it means, especially now with seeing what you’ve said about “previously”. Am I unable to pursue Shannon romantically while on Lana’s route until I’ve successfully triggered her route and did it?
Yes, to romance Shannon in Lana's route you'll need to have completed at least one of Shannon's romantic endings in her own route first. If you're having trouble getting onto Shannon's route, there's a walkthrough available here!