We have no idea what your answer is, so we can't be sure that there were no mistakes. The sentence seems alright, so the suggested translation should be accepted.
There might be a keyboard mix. Since most of Greek capital letters seems exactly the same as the English ones, sometimes users forget to switch keyboards until the lowercase appears, and the capital letters remain unchanged. The sentence will be marked wrong, even if there's only one letter that's not in the correct language. Please make sure that nothing like that might be going on. ^.^
If you feel like the problem remains, please feel free to send us a screenshot of your answer marked wrong, so that we can tell if it's a bug or not. ^.^