It's 無くす, but it's often written in Hiragana なくす actually. 無くす is for one's personal effects such as 傘, wallet, smartphone, key, glasses. 無 itself means "nothing", so 無くす has like a "vanish" nuance. It feels like things that once have existed disappeared in front of your sight. 失くす rather said/written 失う, is used for abstract things such as a position, title, approval rate, or important things other than one's personal effects that you can carry with you such as property, house, a lover or friend, people important to you. This kanji 失 has a "lost" nuance more than 無.

https://ja.wiktionary.org/wiki/%E3%81%AA%E3%81%8F%E3%81%99 なくす【無くす・失くす・亡くす】 1 (無くす・失くす)今まで持っていたものを失う。 2 (無くす)努めて、ない状態にする 3 (亡くす)近親者に死なれる。 They mean the same thing. "To lose something you had."