lack of 英文 (急)

They have a lack of confidence.
Lack is an uncountable noun in the above sentence.

Normally we don’t put “a/an” in front of the uncountable noun. With certain uncountable nouns – especially nouns referring to human emotions and mental activity – we often use a/an when we are limiting their meaning in some way.
For example:
a distrust of politicians, a first-class knowledge of French, a good education, a good understanding of Quantum Mechanics, a good sleep, a longing to get away, a help,
a love/dread/dislike/ hatred of….

Some nouns for feelings are singular as compliment. (e.g. pleasure, delight, delight, relief, shame, wonder)
Thanks a lot for your help. ~ It is a pleasure.
It seemed a pity to break up the party.

Somehow we can make some uncountable abstract nouns countable if we refer to a specific type of the noun.
This is a cheese I really like. (= a kind of cheese.)

題外:
“a lack of confidence” is definitely NOT a noun compliment.
Compliment needs a linking verb. Have is not a linking verb.

Have can be both a main verb (state or action) and an auxiliary verb.
I have a car. I have confidence. (have ~ a transitive verb.)
Have means possess, own, have got.

Some linking verb: be, seem, appear, look, taste, sound … etc
He is my boss.
She was pretty.
The food tastes delicious.

They------subject
have------verb
a lack of confidence-------n.complement
All the complement mentioned has been complement of the subject.they are called noun complement to complete the predicate.

The n.complement
=a lack of (self) confidence
=a shortage of faith.

How about treat it as a verb?
They should have confidence in
=verb=trust.

As you said that "lack" is not a verb,not to be used with "a"..