This is just my opinion, so please take it with a grain of salt. Rather than practicing your pronunciation with the Duo, I think it is better to dictate to Google Voice, and see if it can produce the characters for what you tried to say. If it does or mostly does, then that's plenty good enough. At least you'll see some feedback about what character(s) you may have missed
However Google Voice is not perfect either. It will at times mis-transcribe native Mando speakers as well
块 in Chinese usually is a wide signifier for currency in general, not applied to just the Chinese yuan. Other currencies should be accepted as valid answers.
While there are hundreds of currencies in the world, adjusting this for the more common referrals like the dollar would go a long way.
I have no idea what went wrong with this but it did not accept my answer over and over again until it ate up all my hearts. I know for a fact I was speaking correctly, I speak Mandarin relatively well, my Chinese husband even spoke into the phone. It was always the last part of the phrase that wasn't registering. That kinda made me mad that I had to use up hearts and gems over one stupid phrase and now can't move on when it's obviously not our issue.