"なくす” is the Japanese verb (in the infinitive form) for "to lose (something)." To make the verb negative, you have to change the stem of the verb from nakusu to nakusa and add "nai de kudasai" to mean "Please do not lose (this/it)." A lot of verbs in Japanese form the negative this way, such as kaku - kaka-nai (to write), naosu, naosa-nai (to fix), yakusu, yakusa-nai (to translate).

なくす　-> なくさない　(present negative form)