Her route is locked behind everyone else's. You'll have to play through each route and get both the good and bad endings for all of them before Lien even becomes available. You'll know that you've gotten the good and bad endings for the other characters when you've gotten five collectables on your home page.
By the way, Xanthe's route is also locked behind Lyell, Bae, and Victoria's routes so don't try to romance her before them. Then you need to select the right options in the early game. Looking through the original Monster Sweethearts' comment section will give you the exact choices to make to get Lien's route, but I'd really suggest trying it yourself at least a few times before giving up and looking for the answer. You can figure it out for yourself if you pay attention to the consequences and results of choices while romancing the other characters.
Hope this helped, have fun!