It would not be a completely correct translation. "én" and "en" are two different words. "én" is a numeral (="one") and "en" is an article (="a/an"). The original sentence used the article, not the numeral.
"Én time senere" is one of the accepted answers in the course. But it seems Duolingo has a difficulty handling situations where something can either be an acceptable typo or an alternative correct answer.