This letter is a capital “i” not small “L”
Also the number should be after the chain, not before it, my wrong. You should also fix the interface and remove the “vlan 20”,
Eventually the command should look like that:
iptables -t nat -I POSTROUTING 1 --src 172.30.0.0/16 -o eth1.20 -j MASQUERADE
Aw, gotcha. Is this comman something that is run from the shell?