をなくす

to lose (something)

→ を
phrase Common #2,804
to lose (something) The particle を marks the thing that is lost. なくしてしまう often adds a sense of regret or that the loss was unfortunate.
JA: 財布をなくすと困る。
EN: It’s a problem if you lose your wallet.

トムはよく物をなくす。

Tom often loses things.

君は鍵をなくすなんて不注意だったね。

It was careless of you to lose your keys.

健康を害することはお金をなくすことより重大である。

Damaging your health is more serious than losing money.