Should I be able to put "wnewch chi fy ffonio fi" instead?
Yes, and that is one of the accepted answers.
Odd, I copy-n-pasted that from my answer marked incorrect (not typo)
If you use the Duo app, check that you have the latest version - there are frequent updates and bug fixes. Otherwise, please raise a 'Duolingo bug report'.
No, I never use the app anymore. The web versions are much better. Very odd.