Translation:I want somebody to offer me a drink.
I believe what might be even closer to the fact of the matter is that the program has proven itself to all the users as deeply flawed. Italian is a complex language and the apparatus that is supposed to comprehensively contain it in all its nuance does a second rate job of it at best. In a word, it's Clunky... Attempting to learn a language from a person whom you know speaks it only nominally fluently themselves would be the same. One would become wary of their corrections. I don't need to be right. I just need to be certain I'm wrong. And I have seen too many native speakers say in these forums that the sentence in question is never spoken the way that it's written, or, just as often, that there are two or three other ways to say the same thing that Duo doesn't accept.
You're right, "I want someone that offers me a drink" is not correct and has now been fixed. Your alternative should be "I want that someone offer me something to drink," however, since "offer" is in the subjunctive. Or you could go with the much more natural, "I want someone to offer me something to drink."
DL seems to be insisting on the subjunctive in English, although we often use the indicative in normal English and DL usually accepts both.
For example: "I insist that he leave" and "I insist that he leaves" are both possible in English, but the former (subjunctive) sounds more formal.