The demandingness objection is a philosophical objection to certain things that people might be asked or expected to do. It basically means that some things are too hard or too much to ask of people - they are too demanding. For example, you might have a teacher who is really hard and wants you to do really difficult things like memorize many difficult facts. This could be seen as too demanding and the demandingness objection would mean that the teacher shouldn't keep asking you to do these difficult things.