Board Thread:Questions and Answers/@comment-31718576-20170411140100/@comment-31513590-20170412005246

Mabian wrote: I don't know what is causing the problem. I can't quite gather from your posts if you've performed any edits to the game's files already, or if this is just a bug you've happened to run into. Unfortunately, I also have no clue how to fix it.

If you decide to reset, I should note that you don't need to do everything over again. You just need to befriend Papyrus and Undyne and get past Mettaton (in any order you wish). The game has already registered your save file as having beaten Photoshop Flowey, which is the flag that is needed to trigger Undyne giving you the letter. What he said