C is a programming language. It can be used to develop applications that run on a variety of operating systems, like Windows, OSX, Linux, Unix, Solaris, HPUX, SunOS, mainframes, etc.
Excel is one such application, written for one specific operating system (Windoze).
That code runs on the Arduino. It can NOT possibly display the error message that you quoted. You should avoid using pronouns like "it". Use nouns, like 'the Serial Monitor" or "the Excel application".
On the other hand, the arduino programming window says at the low rigth corner "Arduino Due (Programming port) on COM3". The windows hardware control panel says that the programming port is COM16 and the native port -that I've just connected- the COM20.
The Due has two ports - one for programming and one for serial communication. It's important to use each for it's intended purpose.
You aren't trying to have you C application program
the Due, are you?