作为名词，lack既可以是单数名词，也可以是不可数名词。故 lack of sth 或 a lack of sth 均可。没有多少差别。

《韦氏高阶》lack

: the state or condition of not having any or enough of something :the state or condition of lacking something

[singular]

• The problem is a lack of money.

• a lack of experience

• She has been suffering from a lack of sleep lately.

[noncount]

• Her problem is lack of sleep.

• There was no lack of interest in the proposal. [=there was much interest in the proposal]

• His honesty, or lack thereof, is the real problem. [=the real problem is his lack of honesty]