Board Thread:Questions and Answers/@comment-40185448-20190717180624/@comment-26907577-20190717190730

You cannot accomplish a True Pacifist Run unless you have at least one Neutral under your belt. If you spared everyone, you can reload your save, and go hang out with Papyrus and Undyne. Once you've done that (or if you already have) Undyne should call you when you're on the bridge behind the MTT resort with the quest to initiate TP.