"You are standing."
Translation:तू खड़ा है।
Note that त is an unaspirated 't'. In romanized form, you'd represent तुम without the 'h', i.e. 'tUm', not 'thUm'.
In a way, you're right. त is dental as compared to the retroflex ट. I.e. you pronounce त with your tongue touching your teeth in about the same place you'd use for an English 'th'. But when romanizing, adding an 'h' after another consonant always signifies adding aspiration.
खड़ा/खड़ी/खड़े is an adjective.
The nearest equivalent Hindi verbs to English's "to stand" and "to sit" refer to the act of changing position, so in Hindi you're only (verb) standing or (verb) sitting for a moment. And once that moment has passed, you are (adjective) standing or (adjective) sitting.
तुम always takes plural verbs and adjectives, even when addressing an individual. The plural forms of "standing" are खड़े (masculine) and खड़ी (feminine).
Duolingo's heuristics to detect typos aren't perfect and will sometimes call grammar errors typos or call minor typos outright wrong. We have the former case here. :-)
If I remember, next time I run into this I'll give the answer तुम खड़ा हो and report it as "my answer should not be accepted".