to not have or not have enough of something that is needed or wanted:
He just lacks a little confidence.
What we lack in this house is space to store things.
We are lacking three members of staff due to illness.
lack of sth the absence of something or when there is not enough of it:
Her only problem is lack of confidence.
Lack of sleep had made him irritable.
If he fails it won’t be for/through lack of effort (= he has certainly tried).
We won’t be going on holiday this year – lack of funds, I’m afraid.