It's actually impossible to get the True Pacifist Ending your first time playing through, and it's justified in game. You have to get the neutral ending first. Even if you don't kill anyone during that playthrough, you'll get a neutral ending still. The difference is you don't have to play through it again, it'll just drop you at The End and tell you to go find Alphys.
So I've heard.
It would've been nice to just go find Alphys, but I actually didn't really mind replaying the whole game again.I'm considering doing the Genocide route to get more perspective of the world of this game because I'm a nerd, and also for the sake of completion. I considered watching a playthrough on Youtube, but I get a more personal experience when I play it. But we'll see.
Which songs do you like?