What is the difference between "a lack of confidence" and "lack of confidence?"

The presence of an article simply converts a noun from non-countable to countable.

And "lack" is one of those words that can be used countably or non-countably - almost interchangeably.

However other similar nouns behave differently. Take for example "supply". One could say "They needed a suppply of onions", or ..."supplies of onions". But one would less frequently say "They needed supply of onions". But the last is not entirely ruled out.

Others, such as "abundance" would almost always require an article. "There had been an abundance of tomatoes this year" but rarely "there had been abundance of...". Though one could, of course say "tomatoes are abundant".

My attention has been drawn by the redoubtable @Edwin Ashworth to the null article which undoubtedly has great significance here.