sorry, I am trying to be helpful, I didnt actually run it with --help I just looked at the install script which clearly shows that it would display info that would help you see that you need to add --testnet .. but seemingly that's not what you're seeing when you actually execute it.
usage() {
echo " [+] Install script usage:"
echo
echo " [sudo] bash install-sx.sh [<--argument> [...]]"
echo
echo " If no arguments are provided, defaults will be used, and sudo is mandatory."
echo
echo " Default path for installation is $INSTALL_PREFIX"
echo " Default path for the conf files is $CONF_DIR"
echo " Stable versions of toolchain packages (from git $TOOLCHAIN_BRANCH branches) will be installed for libbitcoin, libwallet, obelisk and sx tools."
echo
echo " Optional arguments:"
echo " --prefix