"What are you trying?"
Curious response: https://screenshots.firefox.com/brP1lVC6QCbN98SU/www.duolingo.com. Why is τι underlined? I chose a different verb and person, but it accepted them as correct (which they are) but claimed a typo for a two-letter word which is not misspelled ...
"Τι προσπαθείς." is exactly one of the correct answers. And that is shown in your screenshot.
But you also should know that the mods have no control over these technicl errors. It has to be reported to Duolingo Staff which I will do and many thanks for the screenshot.
Ok, I sent your screenshot and one of our incubator showing the exact sentence to for the technical staff to review. Thank you.