lack of experience
a lack of parents
lack of attention
is it correct? what is the rule(lack / a lack)?
You need to use these phrases in sentences in order to get a better answer.
However, in general, lack as a noun requires an article (there are exceptions, but generally... you need a or the with it). It also is usually followed by of and a noun, such as "Bob really showed a lack of confidence in last night's game.")
Lack as a verb (as in, "Susan lacks experience.") does not require an article, of course.