Th used to be pronounced as in English thing but it isn't anymore. It is now /h/, or silent when no vowel follows.
Thu is an exception and I don't think anyone knows why. But then I don't think anyone knows why it has an h in it anyway. It doesn't in Irish (except when you would expect lenition) so it is doubly weird. D
It's very easy to make mistakes without noticing, and Duolingo does have genuine bugs as well. But unfortunately it not possible to diagnose or fix without more detail. So next time this happens, please do two things:
- Copy and paste your wrong answer and their 'correct' answer here
- Say which bits it underlined - these are the bits that it alleges are wrong
This probably isn't possible on the app, and might not even be possible on a phones, but these small details are really important with the diagnosis. (If anyone knows if you can do this on an app that would be great.)