Where to start fixing error at <<none>>:0:

I’m getting the following error when running nix flake check .

warning: Git tree '/home/mrhappy200/persist/mynewconf' is dirty
warning: unknown flake output 'homeManagerModules'
       … while checking flake output 'nixosConfigurations'

         at /nix/store/1ikib9bbqhw2hcczikj7bkyym8lq51mg-source/flake.nix:73:5:

           73|     nixosConfigurations = {
             |     ^
           74|       # Main desktop

       … while checking the NixOS configuration 'nixosConfigurations.HappyPC'

         at /nix/store/1ikib9bbqhw2hcczikj7bkyym8lq51mg-source/flake.nix:75:7:

           74|       # Main desktop
           75|       HappyPC = lib.nixosSystem {
             |       ^
           76|         modules = [stylix.nixosModules.stylix ./hosts/HappyPC];

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: list index 0 is out of bounds

       at «none»:0: (source not available)

Where do I even begin with this error?
This is my flake

I’d start with: (stack trace truncated; use '--show-trace' to show the full trace)

(I don’t see Built-in Functions - Nix Reference Manual in your code, but iirc that’s where this comes from. It’s likely in a module you’re using. Maybe a the module assumes you’ll add something to the list?)