The purpose of _.pluck is to retrieve a field for each object in a collection - since _.max returns only a single object, you don't need to pluck, you can simply retrieve the field from the one object you have:
var maxScore = _.max(peopleList, scoreLabel)[scoreLabel];
The above will retrieve the person from peopleList who has the largest scoreLabel, and then retrieve that person's scoreLabel value.
Alternatively, you could swap the order of the calls to _.max and _.pluck, like this:
var maxScore = _.max(_.pluck(peopleList, scoreLabel));
This will build a collection of all the scoreLabel values, and then retrieve the largest one.