Checking Oxford and MacMillan dictionaries, "ignorance" is listed as a mass noun. The same in the Cambridge dictionary. However, there is a sentence from the Cambridge English Corpus:
Is this merely anodyne or does it reflect an ignorance of the medieval equity jurisdictions ?
Also, when I google "an ignorance", I get quite a lot of hits in various documents etc. Is that a mistake? As no dictionary I consulted allow this to be used in singular.