This often happens if you are adding a secondary route, especially with Linux source based routing.
ip route add default via 10.10.10.254 table 10
RTNETLINK answers: Network is unreachable
If that happens you will probably find that it is unreachable because your NIC does not have an IP in the 10.10.10.0/24 range so just assign an IP in that range to your NIC and try again.
ifconfig eth0 10.10.10.254 netmask 255.255.255.0 up