Open System Settings → Privacy & Security → Microphone. Make sure UniFlow Trans is in the list and the toggle is on.
If you want to capture system audio (Zoom/Teams output), you also need Screen Recording permission for the same app — even though we're not recording the screen, macOS uses that permission class for output audio.
Restart UniFlow Trans after granting permissions. If it still fails, quit the app, reopen, and check the Audio tab in Settings — the input dropdown should show your actual device.