Translation:I return to Spain on June twentieth.
So this exercise can be problematic, due to the many way to express the date. The course contributors couldn't predict every possible way. If your answer wasn't accepted, report it!
It's not a judgement on Duolingo's part. They are not saying your way is wrong or that your variety of English doesn't matter. There were quite a few passive aggressive comments like "do you have to translate into American?". In the US, they speak American English. They don't speak "American". Please do not take your frustration out on a whole nation by saying their language isn't English. The guidelines ask you to be respectful of other users. Let's cultivate a positive community here :)
Native US English speaker here, Southern Appalachian dialect, also having lived in Maine, Alaska, and Arizona.
You don't hear the structure of "twentieth June" much here—not unless you're watching BBC America or British TV on PBS anyway (or hanging out with anglophiles). I would say either "June twentieth" or "the twentieth of June". "June twentieth" is probably more common in daily speech, and "the twentieth of June" would be more commonly used in documents or formal speech.
I'm also a native US English speaker and long-time preparer of formal documents. We often SAY June twentieth or the twentieth of June, but in writings, it's June 20 or 20 June. (That is, it should be that way in writing. I'm sure I'm going to hear otherwise on here, and it's true that US casual speech carries over into formal writing far too often!)
This sentence has exceptionally many possible translations, so if you find one that's not accepted yet, please report it.
return - come back - go back - am returning - am coming back - am going back
the twentieth of June - June twentieth - (the) twentieth June - June twenty - the 20th of June - June 20th - 20 June - June 20