"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.
"I to sit on the chair" is not how you would translate the equivalent sentence in Vietnamese.
The only reason we sometimes put "to" in front of verbs is to show ourselves in English that they are verbs when we define them in a dictionary. Many other languages don't have that feature.
If you don't put để in this sentence, it just means "They don't have chairs sit." In English, we put "to" in front of "sit" as a way of establishing the relationship between "chair" and "sit" (a chair to sit on, as opposed to a chair that sits, a chair you actually sit on regularly, or again, "a chair sits").
It is a coincidence that that looks the same as the way we would write the definition of a verb in the dictionary. They're two different things, even in English.