"I do not know the fourth one."
Translation:Toho čtvrtého neznám.
Please help me to understand why my response is not correct. "Toho čtvrtého jednu neznám." Is "one" understood without actually using jednu?
It is understood without translating the "one". From Czech point of view, it's strange why English actually needs it there (probably because of missing declensions - Czech declensions make it often more clear what exactly I mean).
You can say "Toho čtvrtého muže/člověka neznám", but that has better English translation "I don't know the fourth man/person."
PS: You're mixing masculine and feminine numerals here. "Čtvrtého" is masculine while "jednu" is feminine ending.
"Neznám toho čtvrtého" is accepted. Perhaps there was something else in your answer that Duo didn’t like. For troubleshooting, it’s best to either (1) use the report button in addition to commenting, or (2) include a screenshot in your comment, so that the team can see your complete answer. (Reporting is easier.)