The most important lines are
error: while evaluating the attribute 'buildCommand' of the derivation 'home-manager-generation' at /nix/store/kasdiz812i365ikl9piq5f1xn62sy0by-nixos-20.03beta482.b780aefc621/nixos/pkgs/build-support/trivial-builders.nix:7:14:
and
while evaluating the option `home.packages':