My rough and ready solution where it is not easy to find a to bach on a Word keyboard and there isn't one below the answer box in DuoLingo is simply to copy the question, paste it into the answer box, delete all words other than the name containing the to bach, and finally type in the rest of the English answer.
I was feeling expansive and said that Siân said that we were fantastic. I just checked the hover clues and it gives fantastic as one translation. Why was I marked 'wrong'?
Well, it is one of those words that can translated as many fairly similar words in English, depending in the context. The best one is probably just 'excellent'. We'll tidy up the hints shortly.
The Duo database is not the easiest thing in the world to edit and keep consistent, so forgive us the oddities that come up sometimes.
How do we distinguish between "Siân said that we WERE [then] great" and "Siân said that we ARE [continuing to be] great", please? (I think I've read it somewhere, but can't remember where.) Diolch.
(Discussions in my profile have disappeared, and so have emails from DuoLingo, so I hope I can somehow find your answer.)
If you look on the web you should be able out find how to get accented letters on your particular device and operating system. For example, on iOS and Android you will see a menu of accented options if you keep a character key pressed for a short time.
You may need to alter your region settings in order to get ŵ and ŷ - see earlier replies for how to get them on macOS, for example.