By default, standard users are not allowed to access the serial ports on a Debian machine. A workaround is to run that program as root, but this is inherently unsafe. A better solution is to add your unprivileged user account to the dialout group.
usermod -a -G dialout username
This allows an unprivileged account to access the system's serial ports. This is handy with programs such as the Arduino IDE, which require serial port access to program the microprocessors.