Translation:Will you show me what you bought yesterday?
"show me what you have bought yesterday" was my answer too, but now I look at it again, I can see where Duolingo is coming from. The question mark, coupled with "Zeigst du mir", make the statement less of an imperative and more a question. "Show me what you have bought today?" sounds strange
I think the suggested solution "Do you show me what you have bought yesterday?" is the translation word by word. If you translate the meaning, "Show me what you (have) bought yesterday." should be accepted. I don't think native speakers would say "Do you show me what you have bought yesterday?"