"Worf fought Duras."
Translation:DuraS Suv wo'rIv.
The software is not quite sure how to handle Klingon and it would take massive rewriting of the base Duolingo software to fix it. I don't know why it offers both the capitalized version and the non-capitalized version and it is annoying that it will accept either. Hopefully it won't impede your progress too much.