problem in external mode with stm32

hello when i started the externel mode i can generat the the code with no error but when i connect the target i get this

    Error occurred while executing External Mode MEX-file 'ext_serial_win32_comm':
Failed to connect to the target. A time-out occurred while waiting for the connection response from the target. Possible reasons for the time-out:
a) The target is not switched on.
b) The target is not connected to your host machine.
c) The application for the model is not running on the target. You might have clicked the Stop button. If the Run button is not dimmed, click it. Otherwise, click the Build button, which downloads and runs your application on the target.

please any help