Arduino DUE: problem uploading sketch to the board

Hello,
I am having troubles uploading a sketch on my Arduino DUE board; in fact it can't actually upload any sketch, no matter what is written on it, so it obviously not a sketch problem.
Some time ago I succesfully interfaced Arduino and Processing, just to graph data Arduino sent to serial port.

Now whenever I try to upload a sketch on my board (both with processing open AND closed) the following error appears.
I have no idea what to do.

Thanks in advance

processing.app.debug.RunnerException
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:98)
	at processing.app.Sketch.upload(Sketch.java:1713)
	at processing.app.Sketch.exportApplet(Sketch.java:1624)
	at processing.app.Sketch.exportApplet(Sketch.java:1596)
	at processing.app.Editor$DefaultExportHandler.run(Editor.java:2392)
	at java.lang.Thread.run(Thread.java:744)
Caused by: processing.app.SerialException: Error touching serial port '/dev/ttyACM0'.
	at processing.app.Serial.touchPort(Serial.java:110)
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:87)
	... 5 more
Caused by: jssc.SerialPortException: Port name - /dev/ttyACM0; Method name - openPort(); Exception type - Permission denied.
	at jssc.SerialPort.openPort(SerialPort.java:170)
	at processing.app.Serial.touchPort(Serial.java:105)
	... 6 more

Hello, I just would like to make sure I understand, that you have loaded a sketch to the DUE in the past and now can no longer load. Or is it you have never been able to load a Sketch?

I have been succesfully using Arduino for the past 9 months, just after using processing it started to have these troubles.

Hey did you ever get this figured out?

I am using a Due for the first time and am not able to upload and get the same error message as you do. I can remove the Due from the USB port connect an Uno and it uploads via the same port no problem???

Dave