Zanti Man-in-the-middle hack HELP PLEASE!

I've been trying to test my network security but every time I try to use MITM it gives me that error. I'm not sure how to fix it. Can anyone help me out please!!!!!

nc: bind: Address already in use
Java.lang.Exception: nc: bind : Address already in use

This error means that the port/ip combo has already been bound and is currently being used for something else, if it's something like 80 check you're not running another webserver like apache/nginx