"Someday, you will be my wife."
Translation:Jednou budeš mojí manželkou.
Well, there are two possible reasons. 1. Duo is acting up. 2. you had a typo or error somewhere else. Duo in case of an incorrect answer looks into all possible correct answers and picks the one that is closest. Maybe you had "Manzelka" and in such a case "moje' is correct. Hard to tell without a screen shot or an entire sentence.