Ok I just checked quickly... two things:
Probably you are lxc-attaching right? Because attach notoriously doesn't export all paths and variables. It's the same for me. If you go over ssh, and I guess lxc-console as well, it's everything in place tho.
Probably you restored a configuration from another router? Because SFO should not be available for x86_64. See Software flow offloading for containerized OpenWrt
As also @anon50098793 said, that's related to the host kernel.Kernels from major distros (at least Debian based) appear to work well without any specific tuning, at least i had no issues so far. And in any case it's just a warning, should be safe to ignore.