I think in English we drop the "a". Like we drop the "the' in "in autumn, in court , in nature, in space, in trouble" to use some DL examples. Sometimes the opposite is true like 'a hundred' or "a thousand' in English is 'hundred' and 'tusend" in Danish. Sometimes the Danish uses plurals like uddannelserne or eksistenser which simply don't work in English. I don't think DL has properly accounted for this (especially in the English translations), or they simply want to keep it as literal as possible for practice purposes.
The danish sentence is rather unnatural; "viden om noget" - "knowledge about something" is probably what I would have written. You can use "et hundred" and "et tusinde" in Danish, in the same way you use "a hundred" and "a thousand" in English. Uddannelserne <-> the educations and eksistenser <-> existences. I don't see how you can translate those differently nor that they don't work in English.
You just don't say the plural. "Education" is a generic noun, so (from an english perspective) there is little point pluralising it. The state administers "education", occasionally "the education", but never "educations". You could talk about plural singular instances though, for example "the educations of Bob" where Bob is pursuing e.g. an education in Maths and an education in History at the same time.
"Existences" follows the same template. While it is grammatically correct to pluralise the word, it isn't actually done unless you are talking about multiple discrete instances of existence applying to one individual. English considers existence and education as universal properties, so constantly marking them as plural would be redundant.