I am running win 10 get "can not open com port 9" I have tried every thing from reinstalling the program Arduino to reinstalling Win 10 I still get can not open com 9. In device manager Uninstalled reinstalled same reply. I have tried to use Fritzzing program and the same thing. I tried uninstalling the driver and reinstalled, I have reinstalled Win 10 and started from scratch to no avail.If there is an answer to this problem Please let me in on it. I have Boco dollars locked up in multiple paper weights at this point. firstname.lastname@example.org thanks Found the problem. You can add Avast fire wall to the list of com port problems. But you have the option of turning it off for one hour if you chose.