Several things to check using the attached URL.
"$ systemctl status
That returns a tree of all the services that your system is running. Look through that list to see if there are any “serial-getty” services running. If there are you should remove them:
$ sudo systemctl stop serial-getty@ttyACM0
$ sudo systemctl disable serial-getty@ttyACM0
While you’re there look through the list for another troublesome member: ModemManager. This is a program used to dial out to the internet through old-fashioned modems (you know, the kind that go beep beep beep squark squeek squark ) or USB DSL modems (that no one uses any more, we’re all wireless now. Remember the old green “frog” modem from way back when…?). That has a habit of grabbing a serial port as soon as it appears and trying to configure a modem attached to it. While that’s happening nothing else can access the serial port. It’s especially a problem with boards like the Arduino Leonardno or chipKIT Lenny which create a new serial port when they enter programming mode. So unless you really need the functionality that ModemManager provides you should just remove it:"