First, start posting on right section. Sedcond, explain better when do you get this error... if while hosting, if while trying to join any server.
If you are trying to host, something is blocking or not properly routing the UDP packets. Some internet providers deny traffic in some specific ports, and it can be your case, if you are 100% sure that you setup correctly the port forwarding.
LFS requires TCP and UDP protocols enabled to host a server.
But if you are trying to join in a server and get this error, try to connect in another. If it's happening in all servers, try to disable the firewall instead just allowing traffic on it.