"Das ist ein großes Kontra."
"Das ist ein großes Kontra." ist not common use in German. There are 10 hits with Google, the first being this very comment page, then 7 hits for evaluation of travel experiences (www.cia.de) possibly all from the same author, one hit for esotericforum.at and one for spin.de.
So you can only guess what the intended meaning of this odd phrase was. The finds by Google seem to mean "counter-argument" ("Gegenargument" in German) or "drawback" ("Nachteil" in German). I suspect this is an anglicism, a try to port the English saying "pros and cons" into German. You do even come across "pros and cons" literally in German talks or papers pretty often, which I personally find quite unimpressive.
Kontra is also not used for a musical instrument as far as I know, only "Kontrabass" (same meaning in English"), and I haven't heard it abbreviated as "Kontra", maybe as "Bass".
Then there is "Kontrapunkt" (counterpoint) a composition style. Also not abbreviated as "Kontra".
The most common use of "Kontra" I know about, is for card games like Skat or Doppelkopf. Here the meaning is a special term to declare a good hand against a player thus increasing the stake of the game.
Probably deriving from the card game use, some people say "Er gibt ihm Kontra.", meaning he is opposing him emphatically. But no "großes Kontra" here either.
Conclusion: duolingo, please drop this phrase from your exercises.