Why is there a collision error when switching generation?

Home Manager currently installs packages into the user environment, precisely as if the packages were installed through nix-env --install. This means that you will get a collision error if your Home Manager configuration attempts to install a package that you already have installed manually, that is, packages that shows up when you run nix-env --query.

For example, imagine you have the hello package installed in your environment

$ nix-env --query
hello-2.10

and your Home Manager configuration contains

home.packages = [ pkgs.hello ];

Then attempting to switch to this configuration will result in an error similar to

$ home-manager switch
these derivations will be built:
  /nix/store/xg69wsnd1rp8xgs9qfsjal017nf0ldhm-home-manager-path.drv
[…]
Activating installPackages
replacing old ‘home-manager-path’
installing ‘home-manager-path’
building path(s) ‘/nix/store/b5c0asjz9f06l52l9812w6k39ifr49jj-user-environment’
Wide character in die at /nix/store/64jc9gd2rkbgdb4yjx3nrgc91bpjj5ky-buildenv.pl line 79.
collision between ‘/nix/store/fmwa4axzghz11cnln5absh31nbhs9lq1-home-manager-path/bin/hello’ and ‘/nix/store/c2wyl8b9p4afivpcz8jplc9kis8rj36d-hello-2.10/bin/hello’; use ‘nix-env --set-flag priority NUMBER PKGNAME’ to change the priority of one of the conflicting packages
builder for ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed with exit code 2
error: build of ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed

The solution is typically to uninstall the package from the environment using nix-env --uninstall and reattempt the Home Manager generation switch.

You could also opt to uninstall all of the packages from your profile with nix-env --uninstall '*'.