"My guest and I eat dinner together."
Translation:Mi kaj mia gasto vespermanĝas kune.
Hm, that sounds odd ...
1) Did you report it?
(because: this is why the course is still in beta - to give us the opportunity to fine-tune the course before it's really released)
2) Did you actually answer with the exact same sentence, word-for-word?
(if you did: straaange ...
if you didn't: maybe something else made it "better have kune at the start of the sentence"? - say, adding a comma:
"Kune, mia gasto kaj mi vespermanĝas.")
[also: please note that I too am a komencanto, so that may (for whatever reason) have been wrong too]
I don't think it was the lack of a period that did it, as I've sometimes added a period when there shouldn't have been one (e.g. one of those "twenty, thirty, forty, ... "-thingies).
I did read though (after having commented; forgot where), that Duolingo probably have/had some sort of hiccup, leading to marking things wrong even though they weren't (or something like that), which sounds more like the actual reason in this case.