"Did you watch the contest?"
As far as I know, both „oglądać” and „obejrzeć” are accepted here. Duolingo isn't too strict on you here, because in a typical situation it hardly matters whether you form the question with perfective or imperfective aspect. But you do seem to understand the grammatical aspect pretty well.
Besides, often it's not so obvious from English sentences which aspect we should use.
Both translations would be correct, however "Oglądaliście konkurs?" is a more literal translation. "Obejrzeliście konkurs" is probably more like Have you watched the contest? (I am not sure whether this would sound well in English)
Both have similar meaning, but "Oglądaliście?" focuses more on the process of watching (asks if they were watching), while "Obejrzeliście?" focuses more in the fact that they have completed the proces (asks if they have/had watched).