nethserver-cockpit requires nethserver-subscription and the latter requires puppet-agent which is not availible for armhfp (and probably not for aarch64).
Moreover we do not have a subscription model for arm which is a community effort.
Also See:
https://community.nethserver.org/t/nethserver-on-raspberry-pi/2851/178
https://community.nethserver.org/t/release-candidate-iso-7-7-1908-rc1/13482/26