I think either 'A lack of tolerance', or 'Lack of tolerance' would be fine.
(The more I think about the possibility mentioned in my earlier post, the more I don't like it, because 'is lacking' sounds more like a present participle than a gerund. So I take back that suggestion.
I checked out the dictionary for the word "lack" and it indicated that it is an uncountable noun.
OK., when would you use the phrases "a lack of" and "the lack of"? What sets them apart when you are trying to use them in sentences? Would you say that whenever you are using the phrase "a lack of," you can almost surely can use the phrase "lack of"?