Why not ''do you know THIS person''? Thank you for answer :) (I am not native English speaker)
You will meet this over and over in this course. Toho, tamtoho is that. Tohoto, tohohle(colloquial) would be this. See the Tips and Notes of this lesson (Demonstratives) and ask if something is not clear.
"Znáš toho člověka?" also is accepted. If that is exactly what you wrote, you may have run into the intermittent Duo grading bug. Please use the Report button, since that allows us to see your complete answer.