When trying to start µPad you might see a message similar to ‘Cannot open serial port `/dev/ttyS0': Permission denied’. Typically this means that the controller is not connected, or that µPad has detected the wrong port.
You can try unplugging the controller from the computer and reconnecting it. Alternatively you may have to explicitly specify a port (see ‘--port’ or ‘-p’).
This error can also occur if another process is using the port. Check that there are no other processes accessing the port. In particular, there could be an instance of µPad running in the background or on another terminal.