JVC KDLHX500 - doesnt let me eject. Comes with...

Question

Hi I have got a JVC headunit - touch screen
Everything seems to be working fine. So far the CD's play fine and the radio palys fine. All buttons are in working order. When I press the eject button on the top right it comes up with an error message saying Reset P21.
Do you know what this is and what I need to do to overcome this
Thanks