Why do I get an error message about ca.desrt.dconf or dconf.service?

You are most likely trying to configure something that uses dconf but the DBus session is not aware of the dconf service. The full error you might get is

error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name ca.desrt.dconf was not provided by any .service files

or

error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.

The solution on NixOS is to add

programs.dconf.enable = true;

to your system configuration.