Board Thread:Questions and Answers/@comment-26311766-20160710210913/@comment-27701762-20160710214702

Yes, you need to complete a Neutral run before doing a Pacifist run. You won't be able to go to the True Lab until that condition is met.

If you've gone through your Neutral run without killing anything, then you are not required to start over from the very beginning. Instead you can reload your save from before the final boss fight and fulfill the criteria for True Pacifist then.