If you know some Hebrew (about Level 3 in Ulpan La-Inyan), you probably know the word for it's missing - חָסִר (khah-SEHR).
Here's how to say to lack: לַחֲסוֹר (lah-khah-SOHR) or לַחְסוֹר (lahkh-SOHR). This word appears conjugated in Psalms 23: הַשֵּׁם רֹעִי לֹא אֶחְסָר (hah-SHEM roh-EE loh ekh-SAHR) - the Name (God) is my Shepherd, I shall not lack (or "want").
The word gets conjugated normally in the past and future tenses, but it functions only as an adjective in the present tense, with חָסֵר (khah-SEHR) instead חוֹסֵר (khoh-SEHR).
Today's "three-weeks" call to action
Join me today in giving something to someone that is lacking it.