Hi, I am a complete beginner at this and had been stuck on this one for a while and couldn't find anything in the forums to help me (I now realise that I don't even want to use this graph, but as it had defeated me for so long I wasn't going to let that get in the way of things).
I had the same WARNING message as above (in white), but below that in red a whole load of text in red, which included:-
gnu.io.PortInUseException: Unknown Application
at
...etc
Well, I ignored/didn't notice that useful piece of advice and carried on tinkering, until much, much, later I realised that perhaps if I had the Arduino software Serial Monitor open the Processing software would not be able to read the serial port. I closed the Arduino Serial Monitor and hey presto all fine.
I had also been trying to solve this problem without the Arduino connected, thinking that 'Run' in Processing was the same as "Verify/Compile" in Arduino - oh dear.
So two very silly basic errors and lessons learned - just sharing this with all the other novices as I couldn't find anything like this in the forum and it may save at least one person some frustration.