The send operation locally completes without problems, but the network later returns an ICMP packet indicating that the destination is invalid. Subsequent socket operations will report that error state.
How you deal with this message is up to you, depending on what you're doing. The message does indicate that your intended destination is not receiving your packets, so usually you'd want to take that into consideration.
If you're getting this error on the loopback it suggests that you haven't correctly bound the listening socket prior to calling sendto.
NVM, after looking around a bit this is a spurious message in this case. Just ignore it.