When running nixos-rebuild --specialisation on a specialisation not declared in e.g. configuration.nix, such as

nixos-rebuild --specialisation my-nonexistent-specialisation

then you only get

building the system configuration...
Command exited with non-zero status 1

but no sensible error message.

It should best print e.g. --specialisation my-nonexistent-specialisation: This specialisation does not exist.

Tested on NixOS 25.05.