The reason is that those routines do not exist until R2018b and r2019a.
Furthermore, readmatrix and writematrix are for dealing with file io not for serial ports.
You should be using instrfind to look for serial ports. Use serial() to construct a serial port object. fopen() it after configuration. fprintf() to send numeric values as text.
Be careful: fprintf outputs down the columns. If you ask to fprintf
Then it will output in the order 1 4 2 5 3 6. The trick is to transpose() 2d matrices