The problem stems from which translation you chose to use. While "aussi" can mean "also", it doesn't mean that in this situation. In this situation, you would use the "too" translation, since that would be the correct way to write it in English.
On a side note, Duolingo also accepts "as well" for "aussi", because "too" and "as well" are synonymous in English.
Why are the articles dropped for occupations when être is used?
we would say: he is a professor.... not: he is professor
I noticed in an earlier question, 'des' was included before soldats;
Ce sont des soldats (as in a particular group of not in general?) but not included in - Ils sont soldats.
Is there a reason for why un, une or les or whatever absent most times and why would it be correct as in the above example?