Just started Hindi tree and theres an error which means i can't continue :(
In Letters 3, theres a question involving तू being tū, and that being a mistake. There's no reporting option or getting around it.
This is a problem that's being experienced by quite a few people and I think a solution is still being worked on. You can report the issue here (with screenshots if possible) to help them understand it better.
For now, the only workaround, like RuslanPyleglot says, would be to clear the problematic lessons by using Duolingo on your phone's browser or on desktop.