Serial port error in script file

Hello all,

I'm trying to control my arduino due with matlab. The arduino is progammed so that when it receives a letter 'd' on the serial input it will do a cyclus with my stepper motor. The program is working in arduino! When i tried in Matlab it also work, but ONLY if i manually put in the code(command window). When making a new script, i get error's that my COM port isn't found (or the program isn't found)! This is right after i tested the command window (nothing is closed or USB isn't removed)

the matlab code is so simple that you can't do anything wrong : s = serial('COM14','BaudRate', 9600); fopen(s) fwrite(s,'d');

As you can see i didn't closed it (yeahyeah i know :D) with the simple reason it have to be used when needed and doens't need to be closed!

anyone that know what can be the possible reason? Many thanks!