From the lessons tips and notes: "Note that it is unnatural to say, for example, שתי שנים. You should always use the dual form, if one exists." Then shouldn't this be שבועיים ? I put "Both weeks" because of this and it was incorrect.
Prescriptivist dictionaries prescribe שָׁבוּעוֹת according to the tanakh (f. ex. שְׁלֹשָׁ֥ה שָׁבֻעִ֖ים יָמִֽים Dan 10,2) and reduce the vowel only in the Dual (שְׁבוּעַ֫יִם) and the constructs (שְׁבוּעַ־ שְׁבוּעוֹת־). Of course colloquial usage might not follow this.
The Tips section told us to use the dual form if a dual form exists, so this should be shvu’ayim. However for some strange reason, here Duolingo gives the plural form, shavu’ot instead of the dual form shvu’ayim for “two weeks”.