I’m glad to hear the build is running for you, that’s great!
I forgot to mention that the AUR PKGBUILD for ipt-netflow doesn’t currently list aarch64 as a supported architecture. If you edit the arch list to include aarch64 manually, the build should compile, install, and run just fine on the espressobin.
It’s been many months since I’ve started using netflow monitoring on the firewall at this point without any problems whatsoever, so I think aarch64 is a supported architecture for the module - I’ve just commented on the AUR package to request that aarch64 be added, so hopefully the maintainer does so in order to avoid making people add the architecture manually.