Hello Mahmoud Ghorbel,
As Hamza ali has already given you an idea how to deal with loop control parameter. Still for your convenience i will try to repeat it that you need to first drag the Loop Control to the Main layout, Connect the output port of the WDM Mux 8×1 to the first Loop Control input port, Connect the first Loop Control output port to the Optical Fiber input port, Connect the output port of the second EDFA Ideal (connected to the Optical Fiber) to the Loop Control input port.
You may refer to these links for further study. I hope this will help.
Regards
Burhan