"I am not going to buy too many things."
Translation:Je ne vais pas acheter trop de choses.
"Je ne vais pas acheter..." and "Je n'acheterai pas..." are actually both correct.
-Je ne vais pas acheter is like saying "I am not going to buy"
-Je n'acheterai pas means "I will not buy"
They are essentially the same thing in both English and French, so typically on Duolingo we will accept both the "going to" and "will" constructions in both languages. Hope that helps! :)