"I have not seen him since Thursday."
Translation:Od čtvrtka jsem ho neviděla.
I can't answer your question as I have no idea what you have submitted as your answer :))
Sorry my word order was different, if you put od čtvrtka jsem neviděla ho. It tells you that it should be ,,jeho" or his, which makes no sense
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.
It is, but it is unusual. It is possible if you really want to stress it was HIM whom you have not seen and not someone else.
"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?
nominative is čvrtek, čtvrtka is genitive
It is customary to add genitive as the "second" form in some dictionaries. Something like Latin servus, servī masc..