Board Thread:Questions and Answers/@comment-35333099-20181020222645/@comment-31593032-20181021022353

The room key doesn't cancel the true pacifist route, but if you answer some questions I think I can find your problem:

-Did you date Papyrus and Undyne?

-Did you complete the Papyrus date (talk to him in his room)?

-Did you give Undyne water at the water cooler?

-Did you fight Asgore?

-Have you done a neutral run in the past?

I'm assuming you didn't kill, since you got Sans's room key.