I'm not sure I understand why "he" is translated as "jeho" here, instead of as "on". Is it that "zajímat" means more like "to interest" or "to be interesting to", and not so much "to be interested in"? Because it would make more sense to me if "the machine does not interest him?" were an acceptable translation, too.
I think this is an example of a time when you need to let go of the idea that every phrase has an exact translation into another language. I mean, mit rad would be literally translated as "to have gladness" or maybe "to have joy" (I'm drawing on my Russian, I don't know exactly what rad on its own translates to, but the equivalent words in Russian would be something like to have gladness), but "Kateřina doesn't have joy in him" would be a terrible translation of Kateřina ho nemá ráda, because that makes almost no sense in English; we translate it to "Kateřina doesn't like him."
Not everything is going to "make sense" in English, and sometimes the natural subject/object construction of a sentence in Czech will be the opposite to what it is in English. In my experience, attempting to make sense of it in terms of "trying to make it fit English paradigms" is usually unhelpful. If it helps you remember that to be interested in is the opposite way around to what you'd expect in English, great (and if this phrase works both ways, fantastic), but that doesn't mean that a given translation should necessarily be different or more literal just because it makes more sense to an English speaker.
I agree that we frequently have to let go of the idea of exact translation (we wouldn't get very far through these lessons without this mindset). But in this case, "Jeho ten stroj nezajímá?" actually does have a direct translation to English, and that is, "Does the machine not interest him?" This is a valid English equivalent where "him" is in the accusative just like in the Czech sentence.
In these cases where there is a direct translation, it seems wrong to translate an accusative into a nominative, even if they mean the same thing. That would be like saying, "She is seeking him" is the same is "He is being sought by her." They have the same meaning, but the translation is incorrect.
I think it's the same reason there's no preposition in the (very direct) English translation, "Does the machine does not interest him?" In this Czech sentence, "machine/stroj" is the subject and "him/jeho" is the object. The translation given on this page, "Is he not interested in the machine?", means the same thing but it swaps the subject and object which might make you think you need a preposition. Just like in English, you need a preposition ("in") when you say "I am interested in that", but no preposition when you say "That interests me."
"jeho" can have different meanings depending on how it is being used. Here, it is being used as an accusative personal pronoun meaning "him" -- see the tips and notes for pronouns here: https://www.duolingo.com/skill/cs/Personal-Pronouns/tips-and-notes
You'll see that 'jeho' is in the list of pronouns for "on (animate)" in the "Acc. w/o preposition" column. That's how it's being used here -- as the accusative object of the verb zajimat [=to interest]. Czech's word order is flexible, so the "him" part is allowed to go first, but the most direct English translation of the sentence would be something like "the machine doesn't interest him?"
Coming to this as a native speaker of American English who was very confused by this sentence, and seeing that others often become confused as well, let me offer the two cents that made it make sense...
Czech has "object-centered constructions" where what would be the subject in the most natural English sentence is either the direct (accusative case) or indirect (dative case) object in a Czech sentence. Zajímat does this sometimes. Some examples.
Accusative-object-centered Constructions: Bolí mě x. = My x hurts. [literally, "x hurts me."] Zajímá mě x. = I am interested in x. [literally, "x interests me."]
Dative-object-centered Constructions: Chutná mi xfood. = I like xfood. [literally, "xfood is tasty to me."] Líbí se mi x. = I like x. [literally, "x is appealing to me."]
One could argue that such constructions are sufficiently foreign to a native English speaker that they could/should be given their own unit early on. In any case, they exist.
Sorry, cell phones, i'm struggling with. once in a while my "messages" get out "unfinished". I used the report botton already, afterwards i've tried to get an answer in the "forum". Might be i should change my attitude. Anyway i've tried to ask if this sentence would be acceptable, as it focuses on "on" not on "zajímá" what would sound "strange". Thx
"O ten stroj se zajímá on." is possible when saying that it is specifically HIM who is interested in that machine - and not the other guy.
A declarative question "O ten stroj se zajímá on?" would be used when you are surprised that it is HIM who is interested in that machine."HE (and not the other guy) is interested in that machine?"