"I have not seen him since Thursday."
Translation:Od čtvrtka jsem ho neviděla.
That's a Duo bug/feature. It tries to suggest the closest translation to what you have written and there are indeed some variants using jeho.
Nevertheless, if your answer was Od čtvrtka jsem neviděla ho., then it is incorrect. Ho is a clitic and requires a pretty specific position in the sentence.
I suggest you (re)read the tips and notes for Pronouns.
"Od čtvrtka" is nominative, right? (at least that's what https://www.nechybujte.cz/slovnik-soucasne-cestiny/%C4%8Cvrtek tells me - i am SO confused) So the "genitive after od" doesn't count for weekdays?