There's no matching constructor for
UdpEchoClientHelper::UdpEchoClientHelper()
These are the available constructors:
/**
* Create UdpEchoClientHelper which will make life easier for people trying
* to set up simulations with echos.
*
* \param ip The IP address of the remote udp echo server
* \param port The port number of the remote udp echo server
*/
UdpEchoClientHelper (Address ip, uint16_t port);
/**
* Create UdpEchoClientHelper which will make life easier for people trying
* to set up simulations with echos.
*
* \param ip The IPv4 address of the remote udp echo server
* \param port The port number of the remote udp echo server
*/
UdpEchoClientHelper (Ipv4Address ip, uint16_t port);
/**
* Create UdpEchoClientHelper which will make life easier for people trying
* to set up simulations with echos.
*
* \param ip The IPv6 address of the remote udp echo server
* \param port The port number of the remote udp echo server
*/
UdpEchoClientHelper (Ipv6Address ip, uint16_t port);
You should take a look to the tutorials first.