Following a water leak detection the Nowa system automatically closes the valve. To open the valve after water leaks, make sure to dry the sensors. After you dried the sensors you'll be able to press the 'Reset' button. You can now press the "Open valve" button and the valve will open. If the valve does not open it means that the sensors have not been properly dried.
Linked below is the procedure to follow to open the Nowa valve after a water leak detection.