Translation:There is one chair.
Ichi is borrowed from chinese and is used as a standalone word for counting, but japanese people could already count before they got ichi, but they could only count objects. Their original word was hito. Now, both words have the same kanji but the reading differs depending on the context (here, the tsu selects the "hito" reading). This happens a lot in other cases, with other words, too.
btw the chinese reading is usually refered to as on'yomi and the original japanese one is kun'yomi
All of these "Enter what you hear" questions have such an inconsistent answer check. You either require entering all hiragana, a mix of kanji, or only kanji.
For example, this particular question does not accept いすがひとつあります
Please flag such answers as accepted as keyboard users will be forced to remember specific answers versus actually learning and understanding the content.