There is an error in line 166 of the file in oxide/plugins/ NightZombies.cs you must replace the line with the following:

server.Command("halloween.murdererpopulation", on ? config.SpawnSettings.murdererPopulation : 0);

Umm, that's what that line already is. Did you mean something else?

Merged post

Ah I see what you mean, I misspelled "Population" at the end of the variable. Changing it to this will break the plugin, since the variable name in the config is also misspelled. This misspelling is the same in both locations so it doesn't actually matter.