Is your Modbus slave actually residing at address 10 ?
That is not a very common situation. The demo code is also quite confusing with all the different serial port definitions. It would be helpful to provide a link to where you found this code. It would also help if you provided a link to whatever the slave device is, otherwise we can only guess as to what is right and wrong in the code.
As is, we can only see one side of what is a pair of devices. We have no idea of what the slave device hardware actually is and what its modbus behaviors are. The more info you provide increases the likelihood someone can help.
Finally, your code needs to be nested in code tags (the </> icon) so it shows up as:
Your code here