An economics student would like to buy a mini-scooter, but she cannot afford one. Which of the following reasons explain why marketing fails to occur here?

Actually marketing doesn't fail here. The student cannot afford it. This doesn't mean that the scooter manufacturer will bring down the prices of the scooter. The primary aim of the manufacturing / marketing or any business for that matter is to earn profit. This is the primary need of the manufacturer and not the desire to satisfy the scooter owners.

Everyone wants a Rolex, a Ferrari or a private jet but most people can't afford them. This doesn't mean that their marketing has failed. Marketing would fail if it is unable to attract the financially well off / people of the appropriate financial cadre and is unable to convince them to buy the product.