Will it still be possible to complete the Genocide ending?

I'm afraid you have to re-do a run from scratch as you may have aborted it by missing monsters.

Remember that you have to kill a certain amount of monsters before fighting one area's boss:

• 20 kills in the Ruins before fighting Toriel
• 16 kills in Snowdin before fighting Papyrus
• 18 kills in Waterfall before fighting Undyne
• 40 kills in Hotlands/Core before fighting Mettaton

Sparing a single enemy will abort the genocide run, fleeing won't.

Minibosses can count as a kill (the dogs in Snowdin, Glad Dummy in Waterfall, Royal Guards and Muffett in Hotlands, the monsters from the path of the fighter in the Core)

You know you're still on the genocide route if you check save points and have either a remaining monsters count or "Determination" as a response

Good luck and have fun