Before the error if the wrong default desktop was chosen would be:
/nixpkgs-channels/lib/modules.nix:282:11:
Default desktop manager ($(defaultDM)) not found.
which has the string interpolation done incorreclty. Now that is fixed
and it is more user-friendly as:
/nixpkgs-channels/lib/modules.nix:282:11:
Default desktop manager (gnome) not found.
Probably you want to change
services.xserver.desktopManager.default = "gnome";
to one of
services.xserver.desktopManager.default = "gnome3";
services.xserver.desktopManager.default = "none";
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