netcat missing -x option

Looks like you have the "traditional" netcat (netcat-traditional) installed. The -x option is available in the OpenBSD netcat (netcat-openbsd). See also: What are the differences between netcat-traditional and netcat-openbsd? on Ask Ubuntu.

There are multiple implementations of netcat, you probably want the BSD one, because it provides the -x option. In Ubuntu it is available in the netcat-openbsd package.