"Horses have lived here since the sixteenth century."
Translation:Koně tady žijou od šestnáctého století.
"Koně tady žijí od šestnáctého stolet" is also 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.)