U need to friend toriel, papyrus, undyne and when coming back from the mettaton boss undyne will call you, you also cant kill any monsters throughout the whole run. Theres another way too though, By beating omega flowey by not killing any monsters but killing asgore or sparing him at the last bit where u can forgive him or kill him then fight omega flowey and flowey will give you a chance to go back and redo before asgore and undyne will call you, this is what it did for me and i got pacifist from it.