Hello,
I am following this blog post Practical Nix Flakes
$ nix-env -f '<nixpkgs>' -iA nixUnstable
$ mkdir -p ~/.config/nix
$ echo 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf
$ nix profile install nixpkgs#hello
copying path '/nix/store/wkw074f3dvakiy5d006cwh50zdp0pmh4-libunistring-0.9.10' from 'https://cache.nixos.org'...
copying path '/nix/store/rrrr1hmxy2wym96yks0gk15xfh1h424v-libidn2-2.3.2' from 'https://cache.nixos.org'...
copying path '/nix/store/ff88p8pnhdmf8bflzbxldys21djw9dp0-glibc-2.33-56' from 'https://cache.nixos.org'...
copying path '/nix/store/v3m94b0jprhhr24f3b6y34dgmhf5rw8p-hello-2.10' from 'https://cache.nixos.org'...
warning: path '/nix/store/ry66219lvpa8w5kswgx1dnyanag2ik9z-profile' claims to be content-addressed but isn't
error: cannot add path '/nix/store/ry66219lvpa8w5kswgx1dnyanag2ik9z-profile' because it lacks a valid signature