how to fix this problem. when I click at serial monitor, it show that problem.
You can't The problem with serial is, only one program can control a serial port at the same time. And the plotter is kind of separate. So when the plotter is open, no serial monitor.
If you really need to do both at the same time then the PLX-DAQ is a great option with added bonus in that it uses MS excel spreadsheets.
If its something a little more simple then there is this HERE