How do I fight Sans in Undertale?

I keep on trying to fight Sans, except every time I do the Genocide Route, he doesn't ask me if I want a bad time.

How do I fight Sans?

Kill 20 monsters in the Ruins before killing Toriel, kill 16 monsters before killing Papyrus, kill 18 monsters before killing Undyne, and then kill 40 monsters before killing Mettaton. You will know you have killed all the monsters when you get an encounter that says "But nobody came." This link might help some more if you are still confused: https://www.eurogamer.net/articles/2018-09-22-undertale-genocide-run-4873

Its easy, here is the answer, you must START from the begining killing everyone before kill toriel. Then repeat in the next areas.

After waking up in the Ruins, progress with the game as usual until you reach the room with a Save Point in the leaves. This area allows the player to have random encounters with monsters.

Stay in this room and walk around to grind random encounters. Whenever you trigger a random encounter, kill all monsters. As you continue to grind, the random encounter rate will gradually drop until you're only getting around one encounter every minute.

And my personal tip: Alt+Tabbing out of Undertale while moving makes you run in place while tabbed out, letting you do other stuff while the main character hunts for random encounters.

Hope it helps :)

You can't fight sans outside a genocide run.

You may have to walk around a lot in a certain area for a while. After you kill monsters for a while, the game will assume it's a genocide route. The save points will say Number left. Once all monsters are killed, the save points will say "Determination." In red text. Hope this helped.