Board Thread:Questions and Answers/@comment-28963347-20160708071010/@comment-5956954-20160708071303

Because you have not befriended Undyne. If you have befriended Papyrus, had not killed Undyne, and poured water on her when she collapsed in Hotland, head to her house (It's next to Napstablook's house). Papyrus should be hanging outside.

After the date with Undyne, she will give you a letter and tell you to deliver it to Alphys, which is when the game will explain to you what to do thereon.

Hope that helps :)