"They do not have chairs to sit."
Translation:Họ không có ghế để ngồi.
Yes, but you have to add the classifer: "Họ không có những cái ghế để ngồi".
"Những cái ghế" = the chairs. "cái ghế" = the chair. When speaking with a native speaker, they usually drop the classifier unless they are making a point or to avoid confusion with similar sounding words or words that might have different meanings.
Duolingo for Vietnamese is pretty awesome but not perfect. My original sentence "Họ không có những ghế để ngồi" is perfectly understood by native speakers.