Undertale pacifist run without Photoshop Flowey

If Undyne doesn't like you (which is to say, Papyrus reports that she refuses to hang out with murderers), unfortunately you appear to have killed someone at some point. There's not a good way to determine who that was, if they're not one of the major characters, and even if you knew, there's nothing you can do to fix it.

However, this may not be the tragedy you think it is, because it is impossible to get the true pacifist ending until you've earned the neutral ending. If you haven't finished the game at least once yet, go ahead and do so, and then start a new pacifist run.

You once received a phone call from Papyrus, who invited you for hanging out with Undyne. In any case, go to the Waterfall, at Undyne's house, and you will be able to start befriending her.

Also, if you didn't give her water after your fight with her, you invalidated the possibility to befriend her. You will have to start another game.