home-manager
documents the step to create the user profiles as well as per user gcroots as a manual process when not on nixOS.
Make sure you have a working Nix installation. If you are not using NixOS then you may here have to run
$ mkdir -m 0755 -p /nix/var/nix/{profiles,gcroots}/per-user/$USER
since Home Manager uses these directories to manage your profile generations. On NixOS these should already be available.
Still they were already available when I installed home-manager
on my Arch Linux with Nix.
Perhaps it helps to create/adjust folders and permissions?