Looks to be fine on my Pi3:
osmc@osmc-pi3:~$ modinfo iptable_nat
filename: /lib/modules/4.14.26-2-osmc/kernel/net/ipv4/netfilter/iptable_nat.ko
license: GPL
srcversion: 5540E1907C770CB63D5E1EC
depends: ip_tables,nf_nat_ipv4
intree: Y
name: iptable_nat
vermagic: 4.14.26-2-osmc SMP preempt mod_unload modversions ARMv7 p2v8
osmc@osmc-pi3:~$ sudo modprobe iptable_nat
osmc@osmc-pi3:~$ lsmod | grep iptable
iptable_nat 16384 0
nf_nat_ipv4 16384 1 iptable_nat
iptable_mangle 16384 0
iptable_filter 16384 0
ip_tables 24576 3 iptable_mangle,iptable_filter,iptable_nat
x_tables 32768 3 iptable_mangle,ip_tables,iptable_filter
osmc@osmc-pi3:~$ iptables --version
iptables v1.6.0
osmc@osmc-pi3:~$ sudo iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
osmc@osmc-pi3:~$ uname -a
Linux osmc-pi3 4.14.26-2-osmc #1 SMP PREEMPT Thu Mar 15 18:20:26 UTC 2018 armv7l GNU/Linux
osmc@osmc-pi3:~$
Assuming you’re on a Pi2 or Pi3, you could try reinstalling the kernel package:
sudo apt-get update
sudo apt-get install --reinstall rbp2-image-4.14.26-2-osmc
Failing that, we’d need full logs: grab-logs -A